ChatGPT answers, unedited

For a large catalog with frequent back-in-stock alerts, I’d generally prioritize a platform that can:

• track product + variant-level inventory (SKU/color/size)
• store waitlists per product
• trigger emails immediately when inventory changes
• handle high-volume sends without duplicates or stale product data
• integrate with your ecommerce platform and analytics stack

The strongest options are:

Klaviyo Help Center Omnisend Help Center### My shortlist for a large catalog

1. Klaviyo — usually the default choice for a big catalog

• Handles back-in-stock flows using product/variant data and can work with platforms such as Shopify, BigCommerce, Magento 2, PrestaShop, Shopware, SFCC, or custom catalog feeds/API integrations. Klaviyo Help Center Klaviyo Help Center
• Good if you want more than alerts:

• “Customers waiting for this SKU”
• VIP/customer segmentation
• browse abandonment + replenishment
• reporting on revenue recovered by product

2. Omnisend — strong if you want faster implementation

• Has dedicated back-in-stock forms and automations that can notify customers when products return. It supports Shopify, BigCommerce, and WooCommerce integrations. Omnisend Help Center
• Good for teams that want less operational complexity.

For a very large catalog, I’d avoid relying only on a basic “notify me” app

Many Shopify apps can send restock emails, but at scale you’ll usually want:

• a centralized customer profile system
• product/variant event tracking
• suppression rules (don’t notify after purchase)
• inventory thresholds (“only alert when 50 units arrive”)
• batching for high-demand products

Klaviyo specifically supports inventory thresholds and notification strategies for back-in-stock flows, which can help when hundreds or thousands of people are waiting for a popular item. Klaviyo Help Center

A few details would change the recommendation:

1. What ecommerce platform are you on (Shopify Plus, Magento, Salesforce Commerce Cloud, custom, etc.)?
2. Roughly how many SKUs/variants and monthly back-in-stock alerts are you sending?
3. Do you need SMS too, or email only?

For a large catalog with frequent back-in-stock alerts, I’d usually avoid treating this as just an email campaign problem. You want a platform that can:

• track customer interest by SKU/variant (not just product)
• trigger alerts immediately when inventory returns
• handle thousands of products and subscribers
• personalize emails with the correct product image, price, variant, and link
• prevent duplicate alerts or overselling
• integrate with your ecommerce platform and CRM

Here are the strongest options by use case:

My typical recommendation by catalog size

10k–100k SKUs (common ecommerce scale)

• Use Klaviyo + a dedicated back-in-stock capture tool (often Back in Stock or Swym).
• Let the alert tool handle inventory events; let Klaviyo handle segmentation, reporting, and lifecycle marketing.

100k+ SKUs or complex variants

• Prioritize a platform that supports:

• API/webhook inventory events
• variant-level tracking
• catalog feeds
• suppression rules
• queue management

If back-in-stock is a major revenue channel

• A dedicated tool like Back in Stock is often preferable because it is built around the “customer wants this exact item when available” workflow rather than a generic email automation flow. Amp

A good architecture is usually:

Inventory system → Back-in-stock engine → Email/SMS platform → Analytics

rather than:

Inventory system → email blast

because the first approach scales by product and customer intent.

If you share:

1. ecommerce platform (Shopify, Magento, Salesforce Commerce Cloud, custom, etc.),
2. SKU count, and
3. monthly back-in-stock alert volume,

I can narrow this to a specific stack.

For a large catalog with frequent restocks, I’d shortlist Klaviyo first, with Omnisend as the main alternative.

My recommendation: Klaviyo

Klaviyo is particularly strong if your requirement is:

Customer subscribes to Product/Variant A → Variant A comes back in stock → automatically email only the customers waiting for Variant A.

Its back-in-stock flow is specifically designed around this model. Customers enter a waiting queue for the particular item/variant, and Klaviyo releases the notification when that item becomes available. Klaviyo Help Center Klaviyo Help Center

It also handles product data dynamically at send time, so the email can pull the current product image, name, price, etc., rather than relying on stale information from when the customer subscribed. Klaviyo Help Center

For a very large/custom catalog, this is especially interesting because Klaviyo supports inventory-aware custom catalogs and APIs, including inventory quantity at the variant level. Klaviyo Developers Klaviyo Developers

When I'd choose Omnisend instead

Omnisend is a very good choice if you're on Shopify, BigCommerce, or WooCommerce and want a relatively straightforward implementation. Its native Back in Stock automation captures the product/variant and dynamically inserts the relevant product into the notification. Omnisend Help Center

Omnisend also has integrations with dedicated back-in-stock apps, which can be useful if your inventory/waitlist logic is more sophisticated. Omnisend Help Center Omnisend Help Center

Quick decision

One important distinction: don't choose a platform merely because it can send an automated email when something comes back in stock. You want a system that maintains the product/variant-specific waitlist and releases the notification to the appropriate subscribers. That's the architecture I'd prioritize for a large catalog.

If you tell me what ecommerce platform you're using (Shopify, Magento, custom, etc.), approximate number of SKUs/variants, and whether inventory comes from an ERP/WMS, I can recommend the exact stack and architecture.

For a large catalog with frequent restocks, I’d shortlist Klaviyo first, with Omnisend as the simpler alternative.

My pick: Klaviyo

Klaviyo is particularly strong when the alert needs to be product/variant-specific at scale.

• Customers subscribe to a specific out-of-stock product, creating a “Subscribed to Back In Stock” event.
• The flow waits for that particular item to return, then sends the alert.
• Product details such as image, price, and product information are pulled dynamically from the catalog at send time, which is valuable when products change while they're out of stock. Klaviyo Help Center Klaviyo Help Center
• It supports Shopify, BigCommerce, Magento 2, PrestaShop, SFCC, Shopware, custom catalog feeds, and API integrations. Klaviyo Help Center
• For a genuinely large catalog, Klaviyo's custom catalog/API support is a significant advantage because you aren't forced into a one-off workflow for every SKU. Klaviyo Developers

klaviyo.com

Omnisend is the alternative I'd consider

Omnisend has a more turnkey approach. Its native Back in Stock automation supports Shopify, BigCommerce, and WooCommerce, and its email can dynamically display the product that came back into stock. Omnisend Help Center

It's especially attractive if you want email + SMS + ecommerce automation without a lot of configuration. Omnisend also has integrations with back-in-stock tools such as StockIQ that can pass product/variant restock events into its automation system. Omnisend Help Center

omnisend.com

What I'd choose

One important architectural point: don't create one automation per product. You want one back-in-stock flow driven by the product/variant that the customer subscribed to, with the catalog supplying the product data dynamically. That's exactly the model Klaviyo supports. Klaviyo Help Center Klaviyo Help Center

If you tell me what ecommerce platform you're on (Shopify, Magento, custom, etc.) and roughly how many SKUs/variants you have, I can recommend the specific stack—including whether you should use a dedicated back-in-stock app alongside the email platform.

For a large catalog with frequent restocks, I’d choose Klaviyo as the default—especially if you need the automation to understand which exact product/variant each shopper requested.

My recommendation

1. Klaviyo — best overall for a large catalog

Klaviyo has a dedicated back-in-stock flow: a shopper subscribes for a specific product, enters a waiting flow, and the flow pauses until that product is actually back in stock. It supports Shopify, BigCommerce, Magento 2, PrestaShop, Shopware, SFCC, custom catalogs, and API-based inventory feeds. Klaviyo Help Center Klaviyo Help Center

That makes it particularly strong when you have:

• Thousands of SKUs/variants
• Lots of simultaneous restocks
• Different customers waiting for different products
• A need for dynamic product name, image, price, and links
• Custom inventory/catalog infrastructure
• Future plans for segmentation, abandoned cart, browse abandonment, etc.

Klaviyo also supports inventory-aware custom catalog feeds, so you aren't necessarily locked into a standard Shopify setup. Klaviyo Developers

help.klaviyo.com

2. Omnisend — excellent simpler alternative

If your primary requirement is straightforward "customer clicks Notify Me → product comes back → send email", Omnisend is worth serious consideration. Its native back-in-stock automation works with Shopify, BigCommerce, and WooCommerce, and its email can dynamically display the back-in-stock item. Omnisend Help Center

Omnisend also has integrations with dedicated inventory/waitlist apps that pass the specific product/variant and restock event into Omnisend, which can be useful for a large catalog. Omnisend Help Center Omnisend Help Center

support.omnisend.com

What I'd pick

The important architectural point: don't build this as a generic "inventory increased → email everyone" automation. You want a product/variant-specific waitlist event tied to the customer's requested item. Klaviyo's back-in-stock flow is explicitly designed around that model. Klaviyo Help Center

If you tell me what ecommerce platform you're on (Shopify, Magento, custom, etc.) and roughly how many SKUs/variants you have, I can recommend the exact setup—and whether you need Klaviyo alone or a dedicated back-in-stock app feeding it.

If you have a large catalog with frequent restocks, I’d shortlist Klaviyo and Omnisend—and I’d lean Klaviyo if product-level targeting and sophisticated lifecycle automation are important.

My recommendation: Klaviyo

Klaviyo is a strong fit because its back-in-stock system is built around the specific product/variant a shopper requested. A shopper submits “Notify me,” Klaviyo records a back-in-stock event, and the resulting flow can dynamically populate the relevant product information. It supports Shopify, BigCommerce, Magento 2, custom catalog feeds, and API setups. help.klaviyo.com

For a big catalog, that matters because you can build one reusable flow/template rather than creating an automation for every SKU.

Typical architecture:

Product goes OOS → customer clicks Notify Me → save product/variant interest → inventory returns → trigger flow → dynamically insert product → send email

When I'd choose Omnisend instead

Omnisend is particularly attractive if you're on Shopify, BigCommerce, or WooCommerce and want a simpler, more turnkey implementation. Its native back-in-stock automation dynamically displays the product that has returned to stock. support.omnisend.com

It also supports product/variant-level data when integrated with dedicated inventory/waitlist apps, including the product title, URL, image, variant, and price—useful for a very large catalog. support.omnisend.com

Quick decision

Both are currently listed among the leading Shopify apps supporting back-in-stock emails. help.klaviyo.comapps.shopify.com

One important consideration: if you mean “we have thousands/tens of thousands of SKUs and want different email logic/content based on each product,” I would design this around one event-driven flow + dynamic catalog data, not thousands of individual automations.

If you tell me what ecommerce platform you're on (Shopify, Magento, WooCommerce, custom, etc.) and roughly how many SKUs/customers you have, I can recommend the best architecture and compare Klaviyo vs. Omnisend vs. a dedicated back-in-stock tool for your scale.

For a large catalog with frequent back-in-stock (BIS) alerts, you’ll usually want a platform that can:

• Track inventory at the SKU/variant level (not just product level)
• Handle thousands/millions of subscribers
• Trigger sends immediately when inventory returns
• Avoid duplicate alerts when multiple variants restock
• Segment by product, category, customer behavior, and inventory priority
• Integrate with your ecommerce platform and ESP/CDP

The best choice depends on your stack, but these are the strongest options:

Best dedicated back-in-stock platforms

Klaviyo + BIS integration

klaviyo.com

Best for: Shopify/Shopify Plus brands that want BIS + broader lifecycle marketing.

Strengths:

• Excellent email automation engine
• Product-level personalization
• Strong segmentation (VIP customers, high-LTV buyers, category interest)
• Can combine BIS with abandoned cart, browse abandonment, replenishment, etc.
• Handles large catalogs well when configured correctly

Typical setup:

• BIS app captures "notify me" requests
• Inventory event triggers Klaviyo flow
• Email dynamically inserts the exact product/variant

Good fit if BIS is part of a larger retention program.

Gorgias Back in Stock / ecommerce integrations

gorgias.com

Best for: Brands that want customer support + ecommerce workflows together.

Strengths:

• Useful if customers frequently ask "when will this be back?"
• Integrates with ecommerce workflows
• Good for support-driven operations

Less ideal if your main goal is high-volume automated marketing.

Swym Back in Stock Alerts

getswym.com

Best for: Large Shopify catalogs with many SKUs.

Strengths:

• Built specifically for wishlist + BIS
• Handles variant-level alerts
• Good Shopify integration
• Strong for catalogs with many products and frequent inventory changes

Appikon Back in Stock

appikon.com

Best for: Shopify merchants wanting a focused BIS tool.

Strengths:

• Easy setup
• Automated email/SMS notifications
• Good for straightforward BIS programs

Enterprise-scale options

If you have a very large catalog (for example, 50k+ SKUs, multiple warehouses, or millions of customer events), consider:

Bloomreach Engagement

bloomreach.com

Good for:

• Enterprise ecommerce
• Complex segmentation
• Real-time personalization
• Large product catalogs

Salesforce Marketing Cloud

salesforce.com

Good for:

• Enterprise customer data models
• Multi-channel campaigns
• Complex governance

My shortlist by situation

For a large catalog with frequent restocks, I would usually avoid using only a basic BIS app as the "brain." A common architecture is:

Inventory system → BIS capture tool → Klaviyo/Bloomreach → personalized email/SMS flow

That gives you product-level automation without losing customer segmentation.

A few details would change the recommendation:

1. What ecommerce platform are you on (Shopify Plus, Magento/Adobe Commerce, BigCommerce, custom)?
2. Roughly how many SKUs/variants do you manage?
3. Do you need SMS alerts too, or email only?

For a large catalog with frequent restocks, I would usually avoid using only a basic BIS app as the "brain." A common architecture is:

Inventory system → BIS capture tool → Klaviyo/Bloomreach → personalized email/SMS flow

That gives you product-level automation without losing customer segmentation.

A few details would change the recommendation:

For a large catalog with frequent back-in-stock alerts, I’d put Klaviyo at the top of the list.

My recommendation

Why Klaviyo: its back-in-stock flow is specifically designed around the individual product a shopper requested. A shopper subscribes to a product/variant, waits in a dedicated Back in Stock delay, and is automatically emailed when that item becomes available. It also pulls the current product information from the catalog at send time, which is particularly useful when you have thousands of SKUs whose prices, images, or descriptions change. Klaviyo Help Center Klaviyo Help Center

Klaviyo also supports Shopify, BigCommerce, Magento 2, PrestaShop, SFCC, Shopware, custom catalog feeds, and API-based setups. Klaviyo Help Center Klaviyo Developers

When I'd choose Omnisend instead

omnisend.com is a strong alternative if you want something more ecommerce-focused and potentially simpler to operate. Its native Back in Stock functionality supports Shopify, BigCommerce, and WooCommerce, and it can preserve the exact product/variant associated with a signup. Omnisend Help Center Omnisend Help Center

For your use case

If you have tens of thousands of products/SKUs and lots of simultaneous restocks, I'd structure it as:

Catalog/inventory system → back-in-stock subscription → product/variant-specific event → automated email → purchase/conversion tracking

The important thing is not to create a separate email campaign or flow for every product. You want one reusable automation that dynamically knows which SKU the customer was waiting for. Klaviyo's architecture is particularly well suited to this. Klaviyo Help Center

If you tell me what ecommerce platform you're on (Shopify, Magento, BigCommerce, custom, etc.) and roughly how many SKUs you have, I can recommend the exact architecture—and whether Klaviyo or Omnisend will scale better for you.

For a large catalog with frequent restocks, I’d generally choose Klaviyo if your goal is sophisticated, product-specific back-in-stock automation.

My recommendation: Klaviyo

Klaviyo is particularly strong when you have lots of SKUs/variants because its back-in-stock flow is designed around the specific product a shopper requested. It can pull the latest product image, price, and other catalog data at send time rather than relying on stale information from when the customer subscribed. Klaviyo Help Center Klaviyo Help Center

It supports back-in-stock flows with Shopify, BigCommerce, Magento 2, PrestaShop, Salesforce Commerce Cloud, Shopware, and inventory-aware custom catalogs/API feeds. Klaviyo Help Center

klaviyo.com

When I'd choose something else

• Omnisend — a very good alternative if you want a simpler ecommerce-focused platform, especially on Shopify/BigCommerce/WooCommerce. Its native back-in-stock automation dynamically displays the restocked item and tracks back-in-stock demand. Omnisend Help Center Omnisend Help Center
• Dedicated Shopify back-in-stock app + Klaviyo — worth considering if your catalog is very large or you need sophisticated waitlists, variant-level inventory thresholds, multiple locations, SMS/push, etc. For example, Amp says it is built for high-SKU catalogs and integrates with Klaviyo. Shopify App Store

The architecture I'd use

For a large catalog, I'd avoid creating a separate email automation for every product. Instead:

Inventory system → product/variant restock event → Klaviyo back-in-stock flow → dynamically populated email

That lets one automation handle thousands of products and variants.

The important detail is variant-level tracking. If someone requests "Blue / Large," you want the notification tied to that exact variant—not merely the parent product. Omnisend, for example, specifically notes that the customer needs to subscribe to the exact out-of-stock variant. Omnisend Help Center

If you tell me what ecommerce platform you're on (Shopify, BigCommerce, custom, etc.), approximate SKU count, and whether you need email only or SMS too, I can recommend the best stack and setup for your catalog.

For a large catalog with frequent restocks, I’d generally choose Klaviyo as the automation platform, paired with a dedicated back-in-stock capture mechanism if your store needs one.

My recommendation

1. Klaviyo — best overall for a large catalog

Klaviyo is particularly strong when the requirement is “email this customer when the specific product/variant they wanted comes back.” Its back-in-stock flow uses the customer's subscribed product/variant and an inventory-aware catalog, rather than requiring you to build a separate automation for every SKU. It supports Shopify, BigCommerce, Magento 2, PrestaShop, Shopware, SFCC, and custom catalog feeds/API integrations. Klaviyo Help Center Klaviyo Help Center

It can also work with a custom catalog, where inventory quantity is supplied to Klaviyo, which is useful if your catalog isn't entirely native to your ecommerce platform. Klaviyo Developers

2. Omnisend — strong alternative

If you're on Shopify, BigCommerce, or WooCommerce, Omnisend now has native back-in-stock forms and automation. The email can dynamically display the specific product information, and it tracks the back-in-stock demand. Omnisend Help Center

I'd lean toward Omnisend if you want a simpler email + SMS + ecommerce automation setup and don't need Klaviyo's broader customer-data/segmentation capabilities.

3. Dedicated back-in-stock app + Klaviyo — best for very high-volume Shopify catalogs

For a really large Shopify catalog, a dedicated app such as apps.shopify.com can handle the waitlist/stock monitoring while Klaviyo handles customer messaging and segmentation. AMP says it is designed for high-SKU catalogs and supports Klaviyo, Shopify Flow, APIs and webhooks. Shopify App Store

What I'd build

Product goes out of stock
        ↓
Customer clicks "Notify me"
        ↓
Capture exact product + variant
        ↓
Customer enters back-in-stock waitlist
        ↓
Inventory changes from 0 → available
        ↓
Automation identifies interested customers
        ↓
Klaviyo sends product-specific email
        ↓
Customer clicks directly to that variant

The important part is product/variant-level tracking. You don't want a generic "Something you wanted is back!" campaign or a manually maintained segment per product. Klaviyo's back-in-stock flow is specifically designed around this event-and-inventory model. Klaviyo Help Center Klaviyo Help Center

If you tell me what ecommerce platform you're using (Shopify, Magento, BigCommerce, custom, etc.) and roughly how many SKUs/restock emails you send per month, I can recommend the exact stack and compare Klaviyo vs. Omnisend vs. a dedicated back-in-stock tool for your catalog.

For a large catalog with frequent back-in-stock (BIS) alerts, you’ll want a platform that can handle:

• Product-level inventory triggers (SKU/variant level, not just product level)
• High subscriber volumes (“notify me when size M in black returns”)
• Segmentation by product interest
• Email/SMS automation
• Suppression of stale alerts
• Analytics on recovered revenue

The best choice depends on your ecommerce stack and how sophisticated you need to get.

Best back-in-stock platforms for large catalogs

1. klaviyo.com + a BIS app/integration — best for advanced lifecycle marketing

Best if: You already use email marketing seriously.

Why it works well:

• Native-style back-in-stock flows through integrations
• Combines BIS alerts with browse abandonment, replenishment, VIP segments, etc.
• Strong product-level personalization
• Good for large catalogs because customer events can feed into broader automation

Typical setup:
Inventory system → BIS app → Klaviyo event → Product-specific email/SMS flow

Good integrations include:

• Shopify BIS apps
• Custom inventory webhook → Klaviyo event
• Ecommerce platform connectors

2. swym.it — best dedicated BIS solution for Shopify catalogs

Best if: You run Shopify and have many SKUs/variants.

Strengths:

• Built specifically for wishlist + back-in-stock + price-drop notifications
• Handles variant-level alerts (size/color)
• Designed for high SKU counts
• Revenue attribution from alerts

This is often a strong choice for apparel, beauty, home goods, and marketplaces with lots of variants.

3. apps.shopify.com — best simple Shopify-native option

Best if: You want something straightforward.

Pros:

• Easy setup
• Works directly with Shopify inventory
• Low operational overhead

Cons:

• Less sophisticated segmentation and lifecycle marketing compared with a Klaviyo-centered setup.

4. omnisend.com — good Shopify-focused alternative

Best if: You want email + SMS automation without building a more complex stack.

Pros:

• Ecommerce-focused
• Good automation templates
• Often easier for smaller teams

5. Custom event pipeline (ERP/WMS → CDP → ESP) — best for very large catalogs

Best if: You have tens/hundreds of thousands of SKUs or complex inventory.

A common architecture:

ERP / WMS
   ↓
Inventory event ("SKU X available")
   ↓
Customer event database
   ↓
Email/SMS platform
   ↓
Personalized BIS notification

This gives you:

• Real-time inventory triggers
• Regional inventory logic
• Warehouse-aware notifications
• Smarter prioritization (high-margin products, VIP customers, etc.)

My recommendation by catalog size

A few questions would change the recommendation:

1. What ecommerce platform are you on (Shopify, Magento/Adobe Commerce, Salesforce Commerce Cloud, custom)?
2. Roughly how many SKUs/variants do you have?
3. Are you sending email only, or email + SMS/push?

For a large catalog with frequent restocks, I’d make klaviyo.com the default choice.

Why Klaviyo fits your use case

• Product/variant-specific alerts: A shopper subscribes to a particular out-of-stock item, and Klaviyo remembers exactly what they’re waiting for. Klaviyo Help Center Klaviyo Help Center
• Inventory-aware triggering: Its Back in Stock delay waits until the subscribed product is actually restocked, rather than requiring you to build a separate automation for every SKU. Klaviyo Help Center
• Dynamic product data: The email can pull the current product image, name, price, etc. from your catalog at send time—particularly useful when you have thousands of products and details change between signup and restock. Klaviyo Help Center
• Scales across catalogs: Klaviyo supports Shopify, BigCommerce, Magento 2, PrestaShop, SFCC, Shopware, and inventory-aware custom catalog feeds/API integrations. Klaviyo Help Center
• Minimum-inventory rules: You can control how many units need to be available before notifications go out, which is valuable when products frequently come back with only a small quantity. Klaviyo Help Center

Klaviyo vs. Omnisend

omnisend.com is a good alternative, particularly if you're primarily Shopify/BigCommerce/WooCommerce. It has native back-in-stock functionality and automation. Omnisend Help Center

I'd lean:

One important distinction: don't build this as "when inventory > 0, send an email to everyone." You want a waitlist → product/variant association → inventory-restock trigger → notification architecture. Klaviyo's native Back in Stock flow is designed around exactly that model. Klaviyo Help Center Klaviyo Help Center

If you tell me what ecommerce platform you're on (Shopify, Magento, custom, etc.) and roughly how many SKUs/variants you have, I can recommend the best setup and architecture for your catalog.

For a large catalog with frequent restocks, I’d shortlist Klaviyo and Omnisend, with Klaviyo as my default choice if product/variant-level targeting is the priority.

My recommendation

• klaviyo.com — best for sophisticated catalogs

• Built specifically around product/variant-level back-in-stock events.
• Customers enter a flow when they request a restock alert, then Klaviyo waits until that specific item is available.
• Its email variables can pull the current product image, price, name, etc. from your catalog at send time, which is particularly useful when products change while they're out of stock. Klaviyo Help Center Klaviyo Help Center
• Supports Shopify, BigCommerce, Magento 2, PrestaShop, Shopware, SFCC, and custom inventory-aware catalog/API setups. Klaviyo Help Center
• This is the one I'd choose if you have thousands of SKUs/variants and want sophisticated segmentation and automation beyond restock emails.
• omnisend.com — best simpler/cheaper alternative

• Has native back-in-stock forms and automation for Shopify, BigCommerce, and WooCommerce. Omnisend Help Center
• The automation can dynamically insert the back-in-stock product into the email.
• It also supports product/variant context and integrations with dedicated restock apps. Omnisend Help Center Omnisend Help Center
• I'd favor it if you want fast implementation without building a complicated marketing infrastructure.

For your specific use case

If your catalog looks something like:

20,000 products → thousands go out of stock → customers subscribe to individual SKUs/variants → inventory comes back throughout the day

I'd architect it around one automated flow, rather than creating individual campaigns per product:

Customer requests SKU 123 → record product/variant → inventory changes to available → trigger flow → dynamically populate SKU 123 → send alert → remove/complete that waitlist entry.

That way, adding your 20,001st product doesn't require creating another automation.

I'd pick Klaviyo if you're serious about scale and personalization; Omnisend if simplicity and cost are more important.

If you tell me what ecommerce platform you're using (Shopify, BigCommerce, custom, etc.) and roughly how many SKUs/variants you have, I can recommend the exact setup and whether you need a separate back-in-stock app.

For a large catalog with frequent restocks, I’d shortlist Klaviyo first, with Omnisend as the main alternative.

My recommendation: Klaviyo

Klaviyo is particularly strong if your requirement is “when product/variant X comes back in stock, email the people waiting specifically for X.”

• Product/variant-specific alerts: Klaviyo's back-in-stock flow tracks the exact item a shopper subscribed to and waits until that item is restocked. Klaviyo Help Center Klaviyo Help Center
• Large/custom catalogs: It supports inventory-aware catalogs through custom catalog feeds or APIs, in addition to Shopify, BigCommerce, Magento, PrestaShop, Shopware, and SFCC. Klaviyo Help Center
• Dynamic product data: The email can pull current product information rather than requiring you to create a separate automation for every SKU. Klaviyo Help Center
• Good fit for complex inventory: If your catalog is coming from an ERP/PIM/custom inventory system, Klaviyo's custom catalog/API support is a significant advantage. Klaviyo Dev Docs

help.klaviyo.com

Omnisend is worth considering

Omnisend has native Back in Stock automation for Shopify, BigCommerce, and WooCommerce. Its automation can dynamically insert the product that triggered the notification, and its reporting includes product-level back-in-stock demand. Omnisend Help Center Omnisend Help Center

It's especially attractive if you want email + SMS automation with relatively straightforward ecommerce setup.

support.omnisend.com

Which I'd choose

One important distinction: you generally don't want to build one automation per product. The scalable architecture is one back-in-stock workflow, with the subscriber's product/variant stored as event/catalog data. The workflow then waits for that particular item to become available and sends the corresponding product dynamically. Klaviyo explicitly supports this model. Klaviyo Help Center

If you tell me what ecommerce platform you're on (Shopify, Magento, custom, etc.) and roughly how many SKUs/variants you have, I can recommend the exact stack—including whether you need a separate back-in-stock app.

For a large catalog with frequent restocks, I’d shortlist Klaviyo and Omnisend rather than building separate automations for every SKU.

My pick: Klaviyo

klaviyo.com is probably the strongest choice if you want sophisticated product-level personalization and expect your automation needs to grow.

• Customers can subscribe to a specific out-of-stock product/variant.
• A restock creates a Subscribed to Back In Stock event that triggers the flow automatically. Klaviyo Help Center
• Emails can dynamically populate the relevant product information rather than requiring a separate flow per product.
• Klaviyo supports Shopify and BigCommerce natively for the back-in-stock form, plus Magento/custom catalog setups through other configurations. Klaviyo Help Center
• It gives you a good foundation for combining back-in-stock with abandoned browse/cart, segmentation, lifecycle messaging, etc.

Omnisend is the simpler alternative

omnisend.com is particularly attractive if your primary requirement is "someone requests Product X → Product X comes back → automatically email that person."

Its native Back in Stock automation supports Shopify, BigCommerce, and WooCommerce, and the email can dynamically display the specific item being restocked. Omnisend Help Center

It also supports product/variant-level data through integrations—for example, a restock event can contain the product title, URL, image, variant, and price. Omnisend Help Center

What I'd choose

Important: You generally don't want one automation workflow per product. The better architecture is one back-in-stock flow driven by the product/variant event, with the email dynamically rendering whichever product triggered the notification.

If you tell me what ecommerce platform you're using (Shopify, Magento, BigCommerce, custom, etc.) and roughly how many SKUs/variants you have, I can recommend the exact setup and whether Klaviyo, Omnisend, or a dedicated back-in-stock app makes the most sense.

For a large catalog with frequent restocks, I’d use Klaviyo as the default choice—especially if you’re on Shopify.

Why Klaviyo fits this use case

• Product/variant-specific alerts: A shopper subscribes to a particular out-of-stock item, and Klaviyo holds them until that specific item comes back in stock. Klaviyo Help Center Klaviyo Help Center
• Handles large catalogs: Klaviyo can work from inventory-aware catalogs, including custom catalog feeds/API integrations, so you aren't limited to manually creating an automation for every SKU. Klaviyo Help Center Klaviyo Developer Docs
• Dynamic product data: The email can pull the relevant product information at send time, which matters when prices, images, or product details change while an item is out of stock. Klaviyo Help Center
• Beyond email: You can add SMS to the same back-in-stock flow for customers who have opted into SMS. Klaviyo Help Center
• Multiple ecommerce platforms: It supports Shopify, BigCommerce, Magento 2, PrestaShop, Shopware, SFCC, and custom catalog/API setups. Klaviyo Help Center

help.klaviyo.com

My shortlist

Omnisend is particularly worth looking at if you want something simpler: its current back-in-stock functionality supports Shopify, BigCommerce, and WooCommerce, and it can retain product/variant context for the automation. Omnisend Help Center Omnisend Help Center

For your specific situation, I'd choose Klaviyo if you have hundreds/thousands of SKUs and want the system to automatically manage waitlists by product/variant rather than building individual automations.

If you tell me what ecommerce platform you're using (Shopify, Magento, BigCommerce, custom, etc.) and roughly how many SKUs, I can recommend the exact architecture—including whether you need a dedicated back-in-stock app or can do it entirely inside the email platform.

For a large catalog with frequent restocks, I’d prioritize a platform that can track the specific product/variant each shopper requested and trigger the email when that exact inventory comes back.

My top pick: Klaviyo

klaviyo.com is probably the best fit if you want a robust, scalable setup.

• Its back-in-stock flow uses a shopper’s Subscribed to Back In Stock event and then waits specifically for the item they requested to return. Klaviyo Help Center
• It supports back-in-stock flows across Shopify, BigCommerce, Magento 2, PrestaShop, Shopware, SFCC, and inventory-aware custom catalogs/API feeds. Klaviyo Help Center
• For a large catalog, you don't need to create a separate automation for every SKU—the flow dynamically uses the relevant product data. Klaviyo Help Center
• If your catalog is custom, Klaviyo's catalog API/feed can track inventory quantity for individual items. Klaviyo Developers

Strong alternative: Omnisend

omnisend.com is worth considering if your primary goal is straightforward automated restock alerts rather than building a sophisticated lifecycle-marketing system.

Its native back-in-stock automation dynamically inserts the product that the customer requested, and it supports Shopify, BigCommerce, and WooCommerce. Omnisend Help Center It can also segment subscribers based on the specific product or variant they're interested in. Omnisend Help Center

How I'd choose

One important distinction: I wouldn't build this as thousands of separate product-specific automations. Instead, use one back-in-stock flow where the customer's requested SKU/variant is carried through the event and the email dynamically renders that product.

If you tell me what ecommerce platform you use (Shopify, Magento, WooCommerce, custom, etc.) and roughly how many SKUs/variants you have, I can recommend the exact architecture—including whether you need a dedicated back-in-stock app in addition to Klaviyo/Omnisend.

For a large catalog with frequent restocks, I’d strongly lean toward klaviyo.com.

Why Klaviyo is a strong fit

• Product/variant-specific alerts: A customer subscribes to a particular out-of-stock item, and Klaviyo waits specifically for that item to return. Klaviyo Help Center Klaviyo Help Center
• Dynamic product data: The email can pull the current product image, name, price, etc. from your catalog at send time—important when products change while they're out of stock. Klaviyo Help Center
• Large/complex catalogs: Klaviyo supports Shopify, BigCommerce, Magento 2, PrestaShop, SFCC, Shopware, and custom inventory-aware catalogs/API integrations. Klaviyo Help Center
• Inventory thresholds: You can control when an item is considered sufficiently restocked, which helps avoid sending an alert for a product that immediately sells out again.
• Beyond back-in-stock: You get the same platform for abandoned cart, browse abandonment, welcome, post-purchase, winback, segmentation, SMS, etc.

Klaviyo vs. Omnisend

omnisend.com is probably the main alternative I'd consider. It has a native Back in Stock automation for Shopify, BigCommerce, and WooCommerce, including dynamic product information in the notification. Omnisend Help Center

My recommendation: If your catalog is genuinely large and back-in-stock is a significant revenue channel, choose Klaviyo. Its dedicated back-in-stock architecture is particularly well suited to having thousands of products/variants with different customers waiting for different items. Klaviyo Help Center

If you tell me what ecommerce platform you're using (Shopify, Magento, WooCommerce, custom, etc.) and roughly how many SKUs/variants you have, I can recommend the best setup—including whether you should use Klaviyo alone or pair it with a dedicated back-in-stock app.

For a large catalog with frequent back-in-stock (BIS) alerts by product, you’ll usually want a platform that can handle:

• High SKU/variant volume
• Product-level waitlists (“notify me when size/color X returns”)
• Real-time inventory triggers
• Segmentation by product, collection, customer behavior, and purchase history
• Email + SMS (often BIS works better with SMS urgency)
• Suppression rules to avoid sending irrelevant alerts

The best choice depends on your ecommerce stack, but these are the strongest options:

Best overall for large catalogs: klaviyo.com + a dedicated BIS app

Best for: Shopify/Shopify Plus brands with a large SKU catalog

Why it works:

• Klaviyo is excellent for lifecycle automation and segmentation.
• Dedicated BIS apps capture the product/variant event and trigger Klaviyo flows.
• You can personalize emails by exact product, variant, customer segment, and predicted behavior.

Common setup:

1. Customer clicks “Notify me”
2. BIS app stores the product + variant request
3. Inventory returns
4. Event fires into Klaviyo
5. Klaviyo sends product-specific email/SMS

Good if you have:

• Thousands of SKUs
• Frequent inventory changes
• Existing retention marketing

Best native back-in-stock solution: gorgias.com or Shopify ecosystem BIS apps

Best for: Shopify stores wanting less engineering

Advantages:

• Faster deployment
• Product/variant tracking built in
• Handles customer notifications without building workflows

Look at established Shopify BIS apps such as:

• swym.it
• notify-me.io

These are often better than trying to build BIS logic directly inside an ESP.

Best for enterprise catalogs: bloomreach.com

Best for: Large retailers with complex merchandising

Strengths:

• Advanced personalization
• Large-scale customer profiles
• Sophisticated triggers and segmentation
• Omnichannel campaigns

Better fit if you have:

• Millions of customers
• Multiple regions
• Complex inventory systems
• Enterprise CRM needs

Best if you already use Adobe: business.adobe.com

Best for: Enterprise brands already in Adobe Experience Cloud

Strong for:

• Real-time customer journeys
• Multiple channels
• Enterprise governance

My typical recommendation by catalog size

A few questions would change the recommendation:

1. What ecommerce platform are you on (Shopify Plus, Magento/Adobe Commerce, Salesforce Commerce Cloud, custom)?
2. Roughly how many SKUs/variants do you have?
3. Do you need email only, or email + SMS/push?

For a large catalog with frequent restocks, I’d shortlist Klaviyo first, with Omnisend as the simpler/possibly more cost-efficient alternative.

My recommendation: Klaviyo

klaviyo.com is particularly well suited if your key requirement is “when this specific SKU/variant comes back, email the people waiting for this specific item.”

Klaviyo’s back-in-stock flow supports Shopify, BigCommerce, Magento 2, PrestaShop, Shopware, SFCC, and custom inventory-aware catalogs/API feeds. It uses a dedicated back-in-stock delay and dynamic product data, so the email can pull the current product name, image, price, etc. at send time. Klaviyo Help Center Klaviyo Help Center

That matters with a large catalog because you don't want to create one automation per product. You create one reusable flow, and the product/variant context travels with each subscriber.

How I'd compare the main options

Omnisend has native back-in-stock functionality for Shopify, BigCommerce and WooCommerce, and can trigger workflows when a product/variant comes back in stock. Omnisend Help Center Omnisend Help Center

Attentive supports back-in-stock waitlists and can send both email and SMS, with product catalog/API support for custom ecommerce implementations. Attentive Attentive One caveat: its standard back-in-stock journey has limitations around variant-specific triggering, so I'd evaluate that carefully if size/color/SKU-level alerts are critical. Attentive

For your specific use case

I'd choose Klaviyo if you have:

• Thousands or tens of thousands of products/SKUs
• Frequent inventory changes
• Size/color/variant-level restocks
• Large back-in-stock waitlists
• Multiple simultaneous restocks
• A need to personalize the email by product
• Other lifecycle flows you want to build around the same customer/product data

The architecture I'd aim for is:

Product goes OOS → customer clicks “Notify me” → customer is associated with that product/variant → inventory changes to available → automation releases the waiting customers → personalized email goes out → customer exits the waitlist.

That scales much better than creating individual automations for every product.

If you tell me what ecommerce platform you're on (Shopify, BigCommerce, Magento, custom, etc.) and roughly how many SKUs you have, I can recommend the best setup and tell you whether I'd pick Klaviyo, Omnisend, or something else.

Omnisend has native back-in-stock functionality for Shopify, BigCommerce and WooCommerce, and can trigger workflows when a product/variant comes back in stock. support.omnisend.com support.omnisend.com

Attentive supports back-in-stock waitlists and can send both email and SMS, with product catalog/API support for custom ecommerce implementations. One caveat: its standard back-in-stock journey has limitations around variant-specific triggering, so I'd evaluate that carefully if size/color/SKU-level alerts are critical.

For your specific use case

I'd choose Klaviyo if you have:

For a large catalog with frequent restocks, I’d shortlist Klaviyo first, with Omnisend as the simpler/possibly more economical alternative.

My pick: Klaviyo

Klaviyo is particularly well suited if your requirement is “customer asked for Product X → Product X comes back → automatically email that customer about Product X.”

Its back-in-stock flow has a dedicated Back in Stock Delay that waits for the specific product/variant the shopper requested, then sends the notification when inventory returns. It supports Shopify, BigCommerce, Magento 2, PrestaShop, Shopware, SFCC, and custom inventory-aware catalogs/API integrations. help.klaviyo.com

That product/variant-level handling is important for a large catalog—you don't want a generic “something is back” campaign that requires you to manually build segments every time inventory changes.

Omnisend is a strong alternative

Omnisend has native back-in-stock forms and automations for Shopify, BigCommerce, and WooCommerce. Its automation can dynamically include the product that was requested, and it also supports product/variant context in integrations. support.omnisend.com

I'd lean toward Omnisend if your primary goal is straightforward ecommerce email/SMS automation rather than building a broader customer-lifecycle/CRM system.

What I'd choose

One other consideration: don't necessarily buy a dedicated back-in-stock app first. Both platforms now have native functionality, so adding another app can create unnecessary synchronization complexity. Klaviyo can even handle custom inventory-aware catalogs through its API, which is useful if your catalog/inventory system is unusual. developers.klaviyo.com

If you tell me what ecommerce platform you're on (Shopify, BigCommerce, WooCommerce, custom, etc.), roughly how many SKUs you have, and your monthly email volume, I can recommend the specific setup—and whether Klaviyo, Omnisend, or a dedicated back-in-stock app makes the most sense.

For a large catalog with frequent restocks, I’d generally use Klaviyo if you want the back-in-stock system to be part of your broader lifecycle marketing stack.

My recommendation

1. Klaviyo — best overall if you already use it for email

Klaviyo has a native back-in-stock flow that associates each subscriber with the specific product/variant they want, waits until that item is actually restocked, and then sends the notification automatically. It also pulls current product information from the catalog at send time, which is particularly useful for a large catalog where prices/images/product data change. Klaviyo Help Center Klaviyo Help Center

It supports Shopify, BigCommerce, Magento 2, PrestaShop, SFCC, Shopware, custom catalog feeds, and API-based implementations. Klaviyo Help Center

help.klaviyo.com

2. Amp Back in Stock — best if you primarily need a dedicated restock engine on Shopify

If you're on Shopify and the main requirement is high-volume, product/variant-level alerts, Amp is worth serious consideration. It says it's designed for high-SKU catalogs and has delivered hundreds of millions of alerts across 25,000+ brands. It supports email, SMS, and push, plus Shopify Flow and Klaviyo integrations. Shopify App Store

apps.shopify.com

How I'd choose

One important architectural point: don't create a separate email automation for every product. Use one back-in-stock flow, with the customer's requested SKU/variant carried through the event. Klaviyo's back-in-stock delay is specifically designed to hold each subscriber until their particular item becomes available. Klaviyo Help Center

If you tell me what ecommerce platform you're on (Shopify, BigCommerce, Magento, custom, etc.) and roughly how many SKUs/restocks you process per month, I can recommend the specific setup and whether Klaviyo or a dedicated app will scale better.

If you have a large catalog with frequent restocks, I’d strongly lean toward Klaviyo—especially if you want the system to scale beyond basic “item is back” emails.

My ranking

Why Klaviyo is my first choice: its back-in-stock flow is built around the specific product/variant a shopper requested. Someone subscribes to a restock, Klaviyo holds them in a dedicated Back in Stock Delay, and when that particular item becomes available, the appropriate message is sent. It also supports inventory-aware catalogs and APIs, which is useful if your catalog isn't entirely straightforward. help.klaviyo.com

Klaviyo's native back-in-stock functionality currently supports Shopify, BigCommerce, Magento 2, PrestaShop, Shopware, SFCC, custom catalog feeds, and API setups. help.klaviyo.comhelp.klaviyo.com

Klaviyo

When I'd choose Omnisend instead

If you primarily want “customer clicks Notify Me → product comes back → automatically email them” and don't need Klaviyo's deeper segmentation/CRM capabilities, Omnisend is worth serious consideration.

Its native back-in-stock automation supports Shopify, BigCommerce, and WooCommerce, and the email can dynamically display the product that was restocked. support.omnisend.com

Omnisend

For a large catalog, there's one important architectural detail

I would not build this as:

“When inventory changes → search all subscribers → send an email.”

Instead, use a waitlist-per-product/variant architecture:

Out of stock
→ shopper clicks Notify me
→ store records customer + product/variant
→ automation waits for that specific SKU/variant
→ inventory becomes available
→ send dynamic email for that product
→ remove/complete that waitlist entry

That matters when you have thousands of SKUs and frequent simultaneous restocks. Klaviyo's dedicated back-in-stock delay is designed specifically around this model. help.klaviyo.com

My recommendation:

• Klaviyo if this is a significant revenue channel and you want sophisticated lifecycle marketing.
• Omnisend if you want a more straightforward ecommerce automation stack.
• If you tell me what ecommerce platform you're on (Shopify, WooCommerce, BigCommerce, custom, etc.), approximate SKU count, and whether you need SMS too, I can recommend the exact architecture and app stack I'd use.

For a large catalog with frequent back-in-stock (BIS) alerts, the right platform depends on your ecommerce stack, catalog size, and how much personalization/automation you need. The main options fall into a few categories:

For a large catalog with frequent inventory changes, I’d usually prioritize:

1. Dedicated BIS engine + ESP
   • Inventory event triggers:
     • Product X returns in stock
     • Notify subscribers for Product X only
     • Stop sending once purchased
   • Best fit: Yotpo Back in Stock + Klaviyo

2. All-in-one lifecycle marketing
   • Use product feeds, events, and flows inside one platform.
   • Best fit: Klaviyo if you want BIS to become part of broader retention campaigns.

3. Enterprise-scale personalization
   • If you have tens of thousands of SKUs, multiple regions, or millions of customers:
   • Consider Bloomreach Engagement or Salesforce Marketing Cloud.

Key features I’d require for your use case:

• SKU/variant-level subscriptions (not just product-level)
• Inventory webhooks or real-time sync
• Batch throttling (avoid sending 100k emails when a popular item returns)
• Priority queues (VIP customers first)
• Purchase suppression
• Localization/time-zone sending
• Analytics by SKU: “this product generated $X from BIS alerts”
• Integration with your inventory system (ERP/WMS), not only your storefront

If you tell me:

1. your ecommerce platform (Shopify, Magento/Adobe Commerce, BigCommerce, custom, etc.),
2. approximate SKU count,
3. monthly BIS alert volume, and
4. whether you need SMS too,

I can narrow this to the best 2–3 choices.

For a large catalog with frequent restocks, I’d prioritize a platform that handles the product/variant association automatically rather than creating separate flows for every SKU.

My pick: Klaviyo

Klaviyo is probably the strongest choice if email is a major part of your ecommerce stack.

• Product-specific alerts automatically: A shopper subscribes to a particular out-of-stock product/variant, and Klaviyo holds them in the back-in-stock flow until that item is actually available. help.klaviyo.com
• No flow per product: You can use one back-in-stock flow across a large catalog rather than maintaining hundreds or thousands of product-specific automations.
• Dynamic product information: Klaviyo looks up the restocked product from your catalog at send time, which helps ensure the image, price, and other details are current. help.klaviyo.com
• Good ecosystem: It supports Shopify, BigCommerce, Magento 2, PrestaShop, Shopware, SFCC, and inventory-aware custom catalogs/API integrations. help.klaviyo.com
• Segmentation and follow-up: You can combine restock behavior with customer segments, VIP status, purchase history, etc.

Strong alternative: Omnisend

Omnisend is particularly attractive if you're on Shopify, BigCommerce, or WooCommerce and want something more ecommerce-focused/simple.

Its native Back in Stock automation captures the product information and automatically sends the restock notification when inventory returns. support.omnisend.com It also has product-level back-in-stock reporting, so you can see which products have the most waiting demand. support.omnisend.com

For a very large catalog, Omnisend can also be paired with a dedicated back-in-stock app such as StockIQ, which passes product/variant information into Omnisend and lets you segment based on what customers are waiting for. support.omnisend.com

What I'd choose

Important: Don't build this as "one automation per product." The scalable architecture is one back-in-stock workflow → customer subscribes to a specific SKU/variant → workflow waits for that item's inventory event → dynamically renders that product → sends the alert.

If you tell me what your store runs on (Shopify, Magento, WooCommerce, custom, etc.), roughly how many SKUs/variants you have, and whether you're already using Klaviyo/Omnisend, I can recommend the exact setup and whether you need a dedicated back-in-stock app.

If you have a large catalog with frequent restocks, I’d strongly favor a platform that understands product/variant-level inventory events, rather than trying to build generic email automations.

My top pick: klaviyo.com

For a large ecommerce catalog, Klaviyo is probably the best overall choice. Its back-in-stock flow can:

• Capture a shopper against a specific product/variant
• Wait automatically until that particular item is restocked
• Pull the latest product name, image, price, and other catalog data at send time
• Send the alert automatically without creating a separate campaign for every SKU
• Support Shopify, BigCommerce, Magento 2, PrestaShop, Shopware, SFCC, and custom catalog/API setups Klaviyo Help Center Klaviyo Help Center

That last point is particularly important for your use case: you don't want thousands of product-specific automations. You want one automation whose event carries the product/variant someone is waiting for.

If you're on Shopify

I'd compare these three:

omnisend.com also has native back-in-stock functionality for Shopify, BigCommerce and WooCommerce, including product/variant context and automated workflows. Omnisend Help Center

For very high SKU volume, I'd also look at AMP Back in Stock. It specifically markets itself toward high-volume catalogs and supports email, SMS, push, Shopify Flow and integrations with Klaviyo. Shopify App Store

The architecture I'd use

For your situation, I'd aim for:

Inventory system → ecommerce platform → back-in-stock event → Klaviyo → one dynamic email flow

Rather than:

Product A → automation A
Product B → automation B
Product C → automation C
...etc.

Klaviyo's model is essentially "customer subscribed to product X → wait until product X is available → send product X's current information." Klaviyo Help Center

That scales much better as your catalog changes.

If you tell me what ecommerce platform you're using (Shopify, BigCommerce, Magento, custom, etc.), roughly how many SKUs you have, and whether you need SMS too, I can narrow this down to the best 1–2 setups for your catalog size.

For a large catalog with frequent restocks, I’d shortlist Klaviyo and Omnisend, with Klaviyo as my default choice if back-in-stock automation is a major revenue channel.

My recommendation

Why I'd pick Klaviyo: its back-in-stock flow is designed around the specific product a shopper requested. Someone subscribes to a particular SKU/variant, and Klaviyo waits until that item is actually restocked before sending. It also pulls the latest product information from the catalog at send time, which is particularly useful when prices, images, or product details change between signup and restock. help.klaviyo.com

It also supports inventory-aware custom catalog feeds/API integrations, so you're not limited to a tiny catalog or a simple Shopify setup. developers.klaviyo.com

When I'd choose Omnisend instead

If you want something easier to operate and more ecommerce-focused, Omnisend is compelling. Its native Back in Stock automation supports Shopify, BigCommerce, and WooCommerce, and dynamically inserts the product that came back into stock. support.omnisend.com

Omnisend also has integrations with dedicated back-in-stock tools, which can be useful if your inventory/waitlist requirements are more complex. For example, its StockIQ integration can pass product/variant interest and restock events into Omnisend automatically. support.omnisend.com

For your use case specifically

I'd structure it like this:

Customer → "Notify me" on SKU/variant → waitlist tied to that SKU → inventory event → automatic email → product-specific landing page → purchase

The important thing is not to create hundreds or thousands of separate email campaigns/flows—use one back-in-stock automation with dynamic product/variant data.

If you tell me what ecommerce platform you're on (Shopify, Magento, BigCommerce, custom, etc.), roughly how many SKUs you have, and your monthly email volume, I can tell you which platform I'd choose and what the architecture should look like.

For a large catalog with frequent back-in-stock (BIS) alerts, I’d prioritize platforms that can handle:

• product-level inventory triggers (SKU/variant level, not just product level)
• high alert volume
• segmentation (VIPs, regions, customer type)
• email deliverability at scale
• analytics on recovered revenue
• integrations with your ecommerce platform (especially Shopify, Magento, BigCommerce, custom)

Here are the strongest options:

For very large catalogs, I would also look at dedicated BIS apps/services rather than building it entirely inside an email platform:

• Back in Stock — purpose-built for inventory alerts; handles customer subscriptions and automatic notifications.
• Swym — useful if you want wishlists + BIS + price alerts together.

My typical recommendations by scenario

Shopify + 10k–100k SKUs
→ Dedicated BIS app + Klaviyo
Use the BIS app for triggering and Klaviyo for branding, segmentation, and analytics.

Enterprise catalog (100k+ SKUs, multiple regions)
→ Bloomreach or Emarsys
You’ll likely need stronger product-data handling and orchestration.

If BIS is a major revenue channel
Track:

• notification signup rate per SKU
• conversion after alert
• revenue per alert sent
• time-to-purchase after restock
• inventory depth vs. number of subscribers

A common mistake with large catalogs is sending alerts immediately to everyone when inventory lands, causing a product to sell out again before high-intent customers can buy. Prioritizing subscribers (VIPs, earliest signups, high-LTV customers) can improve recovered revenue.

If you share your ecommerce platform (Shopify Plus, Magento, BigCommerce, custom), catalog size, and monthly BIS alert volume, I can narrow this to 2–3 best fits.

For a large catalog with frequent restocks, I’d generally choose Klaviyo—especially if you’re on Shopify.

My pick: Klaviyo

Klaviyo has a purpose-built Back in Stock flow rather than requiring you to build the automation yourself. A shopper subscribes to a specific out-of-stock product/variant, Klaviyo holds them in the flow, and when that particular item comes back in stock, it sends the appropriate product email. It supports Shopify, BigCommerce, Magento 2, PrestaShop, Shopware, SFCC, and inventory-aware custom catalogs/API feeds. help.klaviyo.com

For a large catalog, that's important because you want:

• Product/variant-level subscriptions, not one generic "back in stock" list
• Inventory-triggered sending
• Dynamic product name, image, price, and URL
• A queue of people waiting for each product
• The ability to handle thousands of products without creating a separate automation for every SKU
• Reporting on back-in-stock activity
• Integration with your broader email/SMS lifecycle marketing

Klaviyo can also handle custom inventory feeds, so you're not necessarily limited to a standard Shopify catalog. developers.klaviyo.comhelp.klaviyo.com

Klaviyo Back in Stock documentation

Strong alternative: Omnisend

If you want something more focused on ecommerce automation and potentially simpler to operate, Omnisend is worth considering. Its native Back in Stock automation supports Shopify, BigCommerce, and WooCommerce, and the email can dynamically display the specific product that triggered the notification. support.omnisend.com

It also supports product/variant-based segmentation when integrated with inventory systems such as StockIQ. support.omnisend.com

Omnisend Back in Stock documentation

If you're on Shopify and mainly need restock alerts

I'd consider separating the "waitlist/stock detection" piece from the email platform. For example, Shopify apps such as Back In Stock | Restock Alerts can collect product-specific subscribers and fire alerts when inventory returns, while syncing the audience to Klaviyo.

That can be a particularly good architecture if your catalog is very large or has complex variants/inventory rules.

What I'd choose

Bottom line: If you're already using—or willing to use—Klaviyo, I'd make Klaviyo the central platform. Its back-in-stock functionality is specifically designed around the problem you're describing, including product-level inventory awareness and custom catalogs. developers.klaviyo.comhelp.klaviyo.com

If you tell me what ecommerce platform you're on (Shopify, Magento, WooCommerce, custom, etc.) and roughly how many SKUs/variants you have, I can recommend the best architecture and compare Klaviyo vs. Omnisend vs. dedicated back-in-stock apps for your scale.

For a large catalog with frequent restocks, I’d shortlist Klaviyo and Omnisend, with Klaviyo as my default choice if you need sophisticated product-level automation.

My recommendation: Klaviyo

Klaviyo is particularly strong when you have lots of SKUs/variants because its back-in-stock flow can use the specific product/variant the shopper requested and dynamically pull the current product name, image, price, URL, and other catalog data at send time. It supports Shopify, BigCommerce, Magento 2, PrestaShop, Shopware, SFCC, and inventory-aware custom catalogs/API integrations. help.klaviyo.com

The basic architecture is:

Product goes out of stock → shopper clicks “Notify me” → Klaviyo records the requested product/variant → inventory returns → flow releases the shopper → email dynamically renders that exact product.

That's preferable to creating separate automations for every product.

When I'd choose Omnisend instead

Omnisend is a very good alternative, especially if you're on Shopify, BigCommerce, or WooCommerce and want something relatively straightforward to deploy. Its native back-in-stock workflow captures the product/variant request and automatically sends a dynamically populated back-in-stock email. support.omnisend.com

It also has integrations with third-party inventory/restock tools such as StockIQ, which can trigger workflows based on a specific product or variant returning to stock. support.omnisend.com

Quick decision

One important consideration: don't design this as “one automation per product.” Use one back-in-stock flow driven by the customer's requested product/variant, with dynamic catalog data. That's what makes the system manageable when you have thousands—or tens of thousands—of SKUs. Klaviyo explicitly supports inventory-aware custom catalogs/API feeds for this use case. developers.klaviyo.com

If you tell me what ecommerce platform you're on (Shopify, Magento, custom, etc.), approximate SKU count, and whether you need SMS too, I can recommend the specific architecture and compare Klaviyo vs. Omnisend vs. a dedicated back-in-stock app for your catalog.

For a large catalog with frequent back-in-stock (BIS) alerts, I’d prioritize platforms that can handle high SKU volume, product-level personalization, inventory triggers, segmentation, and deliverability. The best choice depends on your ecommerce stack, but these are the strongest options:

My recommendation by scenario

If you’re on Shopify and have thousands of SKUs:

• Start with Klaviyo + a BIS app/inventory integration. It gives you the best combination of product-level alerts and broader customer lifecycle automation.

If you have a very large catalog (50k–500k+ products) or multiple storefronts:

• Look at Bloomreach or an enterprise CDP/marketing stack. You’ll care more about event pipelines, catalog sync, and throttling than just the email template.

If BIS alerts are your main goal (not full marketing automation):

• A dedicated BIS app can be simpler:
  • Back in Stock
  • Swym
  • Notify Me!

Key features I’d require before choosing:

• Product/SKU-level inventory triggers (not generic “store back in stock”)
• Variant support (size/color)
• Queue management for popular products
• Send throttling (avoid 100k emails at once when inventory returns)
• Real-time inventory sync
• Analytics by SKU (conversion rate after alert)
• Ability to suppress customers who already purchased
• Email + SMS support if you want higher urgency

A few questions would narrow this down:

1. What ecommerce platform are you on (Shopify, Magento/Adobe Commerce, WooCommerce, custom)?
2. Rough catalog size (1k SKUs, 50k, 500k+)?
3. Are alerts mostly email, SMS, or both?
4. Do you already use an ESP like Klaviyo/Mailchimp/Braze?

For a large catalog with frequent restocks, I’d shortlist Klaviyo first, with Omnisend as the strongest simpler/usually more cost-conscious alternative.

My recommendation: Klaviyo

Klaviyo is particularly well suited if you have lots of SKUs/variants and want the back-in-stock system to operate at the product/variant level automatically.

Why I’d choose it:

• Product/variant-specific alerts: a shopper subscribes to a particular out-of-stock item, and the flow waits specifically for that item to become available. help.klaviyo.comhelp.klaviyo.com
• Dynamic product data: the email can pull the relevant product information rather than requiring you to build an email for every SKU. help.klaviyo.com
• Inventory thresholds: you can configure how much inventory needs to return before notifications go out—which is valuable if you frequently receive small replenishments. help.klaviyo.comhelp.klaviyo.com
• High-volume catalog support: Klaviyo supports Shopify, BigCommerce, Magento 2, PrestaShop, SFCC, Shopware, custom catalog feeds, and API-based setups. help.klaviyo.com
• You can also control notification strategy for high-demand products, rather than blasting everyone simultaneously when a small quantity comes back. help.klaviyo.comhelp.klaviyo.com

When I'd pick Omnisend instead

Omnisend is a very good choice if your primary requirement is straightforward back-in-stock automation rather than sophisticated lifecycle marketing.

Its native system supports product variants and automatically inserts the relevant back-in-stock product into the email. It also provides a report showing which products have the most people waiting. support.omnisend.com

Omnisend also has integrations with inventory-oriented tools such as StockIQ, which can trigger workflows when a specific product or variant becomes available and segment customers based on their product interest. support.omnisend.com

What I'd use for a large catalog

One important distinction: I wouldn't build this as thousands of separate automations—one per product. You want one back-in-stock flow that dynamically knows which product/variant each subscriber requested. Klaviyo's architecture is particularly well suited to that model. help.klaviyo.comhelp.klaviyo.com

If you tell me what ecommerce platform you're on (Shopify, Magento, WooCommerce, custom, etc.) and roughly how many SKUs/variants you have, I can recommend the best setup and compare Klaviyo vs. Omnisend vs. dedicated back-in-stock apps for your catalog size.

For a large catalog with frequent restocks, I’d pick Klaviyo—especially if you’re on Shopify, BigCommerce, Magento, or a custom storefront.

My recommendation: Klaviyo

Klaviyo has a purpose-built Back in Stock flow rather than requiring you to build a generic inventory-triggered automation. A shopper subscribes to a specific out-of-stock product/variant, enters the flow, and Klaviyo waits until that particular item is available before sending the notification. help.klaviyo.comhelp.klaviyo.com

It’s particularly strong for a large catalog because it supports:

• Product/variant-specific subscriptions
• Dynamic product information in the email
• Inventory-aware catalogs and custom catalog feeds
• Shopify, BigCommerce, Magento 2, PrestaShop, SFCC, Shopware, and API/custom integrations help.klaviyo.com
• API-based implementation if your inventory system/storefront is custom developers.klaviyo.com

Klaviyo

Runner-up: Omnisend

If you're primarily Shopify/BigCommerce/WooCommerce and want something simpler, Omnisend is worth considering. Its native Back in Stock automation dynamically includes the product that was requested, and it has reporting on which products have the most customers waiting. support.omnisend.com

Omnisend

One important distinction: don't build this as a simple “inventory changed → send email” automation. You want to maintain a waitlist keyed to the specific product/variant, then release those subscribers when that SKU comes back in stock. Klaviyo's dedicated back-in-stock architecture is designed around exactly that workflow. help.klaviyo.comhelp.klaviyo.com

If you tell me what ecommerce platform you're using (Shopify, Magento, WooCommerce, custom, etc.) and roughly how many SKUs, I can recommend the best architecture and whether you need Klaviyo/Omnisend alone or a dedicated back-in-stock app feeding into it.

For a large catalog with frequent restocks, I’d shortlist Klaviyo and Omnisend, with a dedicated back-in-stock app if your inventory logic is unusually complex.

My pick: Klaviyo

Klaviyo is probably the best fit if you want the back-in-stock system to become part of a broader lifecycle marketing stack.

• Handles product/variant-specific waitlists, so someone waiting for a particular size/color can be notified when that exact item returns. help.klaviyo.com
• Has a dedicated Back in Stock flow and delay, rather than requiring you to build the logic from scratch. help.klaviyo.com
• Supports dynamic product information in the email, so one automation can serve a very large catalog. help.klaviyo.com
• Supports Shopify, BigCommerce, Magento 2, PrestaShop, Shopware, SFCC, and custom inventory-aware catalogs/API feeds. help.klaviyo.com
• On Shopify/BigCommerce, its form can automatically appear on products that are out of stock. help.klaviyo.com

When I'd choose Omnisend instead

Omnisend is attractive if you're primarily looking for straightforward automated email/SMS commerce messaging rather than a very sophisticated customer-data platform.

Its native Back in Stock automation supports Shopify, BigCommerce and WooCommerce, and the email can dynamically display the specific restocked product. support.omnisend.com

If your catalog is really large

I'd consider a dedicated back-in-stock engine + Klaviyo. For example, Amp Back in Stock specifically markets itself for high SKU volumes and fast-moving catalogs, and can send email/SMS/push notifications while feeding customer demand data back into your marketing stack. apps.shopify.com

Architecture I'd use:

Inventory → Back-in-stock engine → Product/variant waitlist → Klaviyo → instant email

That gives you a single reusable automation rather than creating a separate campaign for every product.

Bottom line:

• Klaviyo → best overall for a large catalog + sophisticated lifecycle marketing.
• Omnisend → best if you want simpler ecommerce automation.
• Amp/other dedicated BIS app + Klaviyo → best if restock volume/SKU complexity is enormous or inventory behavior is complicated.

If you tell me your ecommerce platform (Shopify, Magento, WooCommerce, custom, etc.), approximate SKU count, and monthly back-in-stock alerts, I can narrow this to the best 2–3 options and compare their likely cost.

For a large catalog with frequent restocks, I’d shortlist Klaviyo first, with Omnisend as the main alternative.

My pick: Klaviyo

Klaviyo is particularly well suited if your requirement is “customer X is waiting for product/variant Y → when Y comes back in stock, automatically email X.”

Its back-in-stock flow:

• Tracks the specific product/variant a shopper requested.
• Waits until that item is actually restocked.
• Sends the notification automatically, using current product information.
• Supports inventory-aware catalogs and APIs, which is important for a large or custom catalog.
• Supports Shopify, BigCommerce, Magento 2, PrestaShop, Shopware, SFCC, and custom catalog/API setups. help.klaviyo.com
• Lets you customize the email with dynamic product data so an old price/image doesn't get accidentally used after a long wait. help.klaviyo.com

Klaviyo back-in-stock documentation

Omnisend is a strong alternative

Omnisend has a native back-in-stock form and automation for Shopify, BigCommerce, and WooCommerce. Its automation can dynamically insert the back-in-stock product, and its reporting shows which products/variants have the most people waiting. support.omnisend.com

That makes Omnisend attractive if you're primarily an ecommerce operation and want a somewhat more turnkey setup.

What I'd choose

One important architectural point: don't build this as a giant automation with one workflow per product. You want one back-in-stock flow driven by the product/variant inventory event, with the subscriber's requested SKU/variant carried through dynamically. That's what makes the system scale when you have thousands of products and constant inventory changes.

If you tell me what ecommerce platform you use (Shopify, Magento, custom, etc.), approximate number of SKUs, and monthly back-in-stock notifications, I can recommend the best setup and compare Klaviyo vs. Omnisend vs. a dedicated back-in-stock app for your catalog.

If you have a large catalog with frequent restocks, I’d make Klaviyo the default choice—especially if you’re on Shopify.

Why:

• Product/variant-specific alerts: A shopper subscribes to a particular out-of-stock item, and Klaviyo holds them in a back-in-stock flow until that item is available. help.klaviyo.com
• Scales beyond Shopify: It supports Shopify, BigCommerce, Magento 2, PrestaShop, Shopware, SFCC, and custom inventory catalogs/API integrations. help.klaviyo.com
• Dynamic product data: The email can automatically populate the relevant product information rather than requiring you to build a campaign for every SKU. help.klaviyo.com
• You can add SMS/WhatsApp: Its current back-in-stock form can collect email, SMS, and WhatsApp preferences. help.klaviyo.comdevelopers.klaviyo.comapps.shopify.com
• Good fit for a large catalog: With a custom catalog/API, Klaviyo can use an inventory quantity field ($inventory_quantity) to determine when products become available. developers.klaviyo.com

What I'd use

For a high-SKU business, I'd actually separate the system into two pieces:

Inventory/store → back-in-stock subscription → Klaviyo → automated email

That gives you one centralized customer/marketing system while the inventory system determines exactly when a particular SKU/variant becomes available.

If you're on Shopify, dedicated apps are also worth considering. For example, Shopify's current App Store has hundreds of back-in-stock solutions, including apps designed around high-volume catalogs and email/SMS alerts. apps.shopify.com

My pick: Klaviyo if back-in-stock is part of a broader retention/CRM strategy. If you're only trying to send restock notifications and don't need a full marketing platform, I'd choose a dedicated back-in-stock app instead.

If you tell me your ecommerce platform (Shopify, Magento, WooCommerce, custom, etc.), approximate number of SKUs, and monthly restock-alert volume, I can narrow it down to 2–3 platforms and compare the automation architecture and likely cost.

For a large catalog with frequent back-in-stock (BIS) alerts, you’ll want a platform that can handle product-level inventory events, high subscriber volume, segmentation, and reliable triggering. The best choice depends on your ecommerce stack (Shopify, Magento, custom, etc.), but these are strong options:

For a large catalog, I’d prioritize these capabilities:

• Variant-level inventory tracking (not just product-level)
• Automatic suppression after purchase (avoid alerting someone who already bought)
• Waitlist throttling (prevent 50,000 emails firing at once after a restock)
• Priority rules (VIP customers first, high-margin products first, etc.)
• Inventory-aware segmentation (different messaging by product category)
• Analytics by SKU/product (which products generate the most recovered revenue)

My typical recommendations:

• Shopify + serious growth brand: Klaviyo + a dedicated BIS app/integration is usually the strongest setup.
• 100k+ SKUs or enterprise retail: Bloomreach or a similar enterprise CDP/marketing platform.
• Need something quick and low-maintenance: Omnisend or a dedicated Shopify BIS solution.

A few questions would narrow this down:

1. What ecommerce platform are you on (Shopify, Magento/Adobe Commerce, BigCommerce, custom)?
2. Roughly how many SKUs/variants do you have?
3. Are these alerts email only, or do you also want SMS/push?
4. How many BIS notifications do you send per month?