ChatGPT answers, unedited

If you're an agency managing multiple ecommerce clients, I'd shortlist Omnisend and Klaviyo, with Omnisend probably the better fit if the priority is simplicity and managing many stores from one interface.

My pick: Omnisend

omnisend.com is particularly agency-friendly for this use case:

• Multiple stores/accounts: You can manage multiple client stores and switch between them without repeatedly logging in. Omnisend supports up to 500 stores under one account. Omnisend Omnisend Help Center
• Email + SMS together: Both channels are built into the same platform and automation workflows, rather than requiring separate tools. Omnisend
• Agency templates: Its Partner Portal lets agencies create reusable campaigns, automations, and signup forms and copy them to client stores. Omnisend Help Center
• Client separation: Each store retains its own contacts, automations, reports, and data, while your team can have different permissions per store. Omnisend Help Center
• Ecommerce-focused: It syncs products, orders, and customer data to power ecommerce-specific email/SMS automation. Omnisend Help Center

When I'd choose Klaviyo instead

klaviyo.com is the stronger choice if your agency works with larger/more sophisticated DTC brands and needs deeper segmentation, analytics, predictive features, and advanced lifecycle marketing.

Klaviyo also supports multiple client accounts under one login, and its Portfolio functionality provides a higher-level view across accounts. Klaviyo Help Center Klaviyo Academy It now has a unified omnichannel campaign builder for email, SMS, push, and WhatsApp as well. Klaviyo

Quick comparison

Bottom line: If you're saying "I want my team to log into one place, jump between 20–100 ecommerce clients, manage both email and SMS, reuse flows/templates, and keep client data separated," I'd start with Omnisend. Its current multi-store and Partner Portal features are unusually well aligned with that exact agency workflow. Omnisend Help Center Omnisend Help Center

If you tell me roughly how many clients you manage (e.g. 10, 50, 200+) and whether they're mostly Shopify, I can narrow this down to the best 2–3 options and compare agency pricing, white-labeling, permissions, reporting, and client billing.

If your agency mainly serves ecommerce brands, I’d pick klaviyo.com.

It’s probably the best fit when the requirement is specifically multiple ecommerce clients + email + SMS + centralized agency access.

My ranking

Why I'd choose Klaviyo

Klaviyo now supports multiple accounts under one login, and its Portfolio functionality gives agencies a high-level view across multiple brand accounts. Klaviyo Help Center Klaviyo Help Center

More importantly, it's built around ecommerce data: customer behavior, purchases, segments, flows, revenue attribution, etc. Its current omnichannel campaign builder can coordinate email, SMS, push and WhatsApp from one interface. Klaviyo

There is also shared billing for eligible groups of accounts, which can be useful when your agency is managing many clients. Klaviyo Help Center

When I'd choose HighLevel instead

If what you really mean by "single interface" is:

I want my agency team to log into one agency dashboard, jump between clients, manage email/SMS/CRM/funnels, white-label the software, and potentially resell it.

Then gohighlevel.com is the stronger choice.

HighLevel is explicitly structured around agency accounts + client sub-accounts, with agency-level permissions and centralized management. HighLevel Support Portal HighLevel Support Portal It also combines CRM, email, SMS, automation, funnels and white-labeling. GoHighLevel

The tradeoff is that Klaviyo is much more ecommerce/retention-centric, while HighLevel is more of an agency operating system.

The simple decision

• 90%+ Shopify/DTC/ecommerce clients → Klaviyo
• Mixed clients + lead generation + CRM + SMS → HighLevel
• Small/mid-sized Shopify clients where cost matters → Omnisend
• Complex B2B automation → ActiveCampaign

For a pure ecommerce marketing agency, I'd actually lean Klaviyo pretty strongly. Current comparisons also continue to identify Klaviyo as the stronger specialized choice for ecommerce agencies, while HighLevel wins on agency-native operations. Tool Decision Engine

If you tell me roughly how many clients you manage (e.g. 10, 50, 200) and whether they're mostly Shopify, I can narrow this down to the best setup—including white-labeling, client billing, permissions, SMS costs, and reporting.

Klaviyo now supports multiple accounts under one login, and its Portfolio functionality gives agencies a high-level view across multiple brand accounts. Klaviyo Help Center Klaviyo Help Center

More importantly, it's built around ecommerce data: customer behavior, purchases, segments, flows, revenue attribution, etc. Its current omnichannel campaign builder can coordinate email, SMS, push and WhatsApp from one interface. Klaviyo

There is also shared billing for eligible groups of accounts, which can be useful when your agency is managing many clients. Klaviyo Help Center

When I'd choose HighLevel instead

If what you really mean by "single interface" is:

I want my agency team to log into one agency dashboard, jump between clients, manage email/SMS/CRM/funnels, white-label the software, and potentially resell it.

Then gohighlevel.com is the stronger choice.

HighLevel is explicitly structured around agency accounts + client sub-accounts, with agency-level permissions and centralized management. help.gohighlevel.com help.gohighlevel.com It also combines CRM, email, SMS, automation, funnels and white-labeling.

The tradeoff is that Klaviyo is much more ecommerce/retention-centric, while HighLevel is more of an agency operating system.

For an agency managing multiple ecommerce clients, I’d put Klaviyo at the top of the list.

Why it fits particularly well:

• Email + SMS in one platform: campaigns, automations/flows, segmentation, consent, and reporting can all work across both channels rather than maintaining separate email and SMS systems. Klaviyo Help Center Subjectlime | Klaviyo Agency
• Multiple client accounts: you can use one login to switch between separate client accounts. Klaviyo Help Center
• Portfolio view: Klaviyo's Portfolio feature gives agencies a higher-level view across multiple accounts, including usage and performance. Klaviyo Academy Klaviyo Help Center
• Good ecommerce fit: it has deep ecommerce integrations and is especially strong for Shopify/DTC businesses.
• Agency-friendly permissions: different users can have different permissions in different client accounts, which is useful when you have account managers, strategists, and execution teams. Klaviyo Help Center
• Shared billing is possible: for eligible related accounts, Klaviyo can consolidate email/SMS usage under a parent account. Klaviyo Help Center

How I'd rank the main options

Attentive is worth considering if your clients are large, SMS-heavy brands. It's particularly strong around SMS, conversational messaging, and omnichannel engagement, but Klaviyo tends to be the cleaner choice when you want email and SMS tightly integrated with ecommerce data. Attentive ATTN Agency

My recommendation: if you're building an agency stack today and most clients are Shopify/DTC, start with Klaviyo + its Portfolio/account-management capabilities. It gives you the closest thing to a centralized agency operating model while keeping each client's data and messaging environment separated.

If by "single interface" you mean literally one dashboard where your team can create/send campaigns for 20–100 different client stores without switching accounts, that's a slightly different requirement—and I'd evaluate agency-focused platforms differently.

For an agency managing multiple ecommerce clients, I’d narrow it to Klaviyo vs. Omnisend:

🥇 Best overall for an agency: Klaviyo

Klaviyo is probably the stronger choice if your clients are serious DTC/ecommerce brands.

• Email + SMS in the same platform and campaign builder. www.klaviyo.com
• You can use one login to switch between multiple client accounts. help.klaviyo.com
• Klaviyo now has agency-oriented capabilities for connecting and analyzing multiple client accounts, including cross-client reporting and audits. help.klaviyo.comsupport.omnisend.com
• Stronger choice if you're doing sophisticated segmentation, flows, attribution, testing, and lifecycle marketing.

🥈 Best if you prioritize agency efficiency: Omnisend

Omnisend is particularly interesting for agencies because its Partner Portal is explicitly built around managing client stores.

It supports:

• Multiple client stores from one login
• Email + SMS + push
• Reusable templates, campaigns, automations and segments
• Templated accounts, where you build an automation/campaign once and copy it to multiple client stores
• Store-specific permissions and billing
• Up to 500 stores under one account in its multi-store setup. support.omnisend.comsupport.omnisend.com

My recommendation

If you're building an agency where one team manages 10–100+ ecommerce brands, I'd seriously look at Omnisend first. Its current partner tooling is unusually aligned with that exact workflow. If your clients are larger brands where sophisticated CRM and revenue attribution matter more than operational simplicity, I'd lean Klaviyo. help.klaviyo.comsupport.omnisend.com

If you tell me roughly how many clients you manage (e.g. 10, 50, 200) and whether they're mostly Shopify, I can also compare Klaviyo vs Omnisend vs Attentive specifically from an agency's perspective, including pricing, white-labeling, permissions, reporting, and client billing.

If you're an agency managing multiple ecommerce brands, and the key requirement is email + SMS from one interface across multiple client accounts, I’d narrow it to three:

My pick: Klaviyo if you're an ecommerce-specialist agency

Klaviyo is the strongest choice if your clients are primarily Shopify/WooCommerce/DTC brands. Its advantage is the depth of ecommerce data, segmentation, flows, product triggers, and revenue attribution, rather than simply being a communication platform. It supports both email and SMS. www.gohighlevel.com

The downside is that it isn't as agency-native as HighLevel—you don't get the same "one agency dashboard → isolated client subaccounts → white-label/resell" model.

Pick HighLevel if the agency experience matters most

HighLevel

HighLevel is probably the better answer if you mean:

"I want my team to log into one agency platform, switch between clients, manage email/SMS/CRM/automations, and potentially let clients access their own branded workspace."

It was designed around sub-accounts and agency management, with email, SMS, CRM, workflows, reporting and white-labeling in the same system. www.gohighlevel.com

The tradeoff is that Klaviyo is considerably more ecommerce-specialized.

Don't overlook Omnisend

Omnisend

This is particularly interesting if you're managing multiple ecommerce stores. Omnisend explicitly supports multiple stores under one account, lets you switch between them without logging in/out, reuse templates/workflows, and manage email, SMS and push together. www.omnisend.com

Bottom line

• Ecommerce retention agency → Klaviyo
• Full-service agency / white-label SaaS → HighLevel
• Lots of ecommerce stores + simple centralized email/SMS → Omnisend

If your real requirement is "50+ ecommerce clients, one agency login, client-level permissions, reusable campaigns/flows, consolidated reporting, and ideally white-label", I'd evaluate HighLevel vs. Omnisend vs. Klaviyo's agency/partner setup specifically rather than generic email platforms.

For an agency managing multiple ecommerce clients, I’d put Omnisend at the top of the list right now.

My ranking

🥇 Omnisend — probably what you're looking for

Omnisend is unusually well suited to an agency model. You can manage multiple stores from one account, switch between them without repeatedly logging in, control user permissions by store, and reuse templates/workflows across stores. It supports email, SMS and push from the same platform. www.omnisend.comsupport.omnisend.comhelp.klaviyo.com

More importantly for an agency, Omnisend added a Partner Portal with templated accounts. You can build campaigns, automations and signup forms once and copy them into multiple client stores—very useful if you're onboarding lots of Shopify/WooCommerce/BigCommerce clients. support.omnisend.com

Omnisend

🥈 Klaviyo — better if your clients are more sophisticated

Klaviyo is arguably the stronger choice if you're managing higher-value Shopify brands that need deep segmentation, sophisticated customer data, advanced attribution, and complex automations.

Klaviyo supports multiple accounts under one login, so an agency can switch between client accounts without logging in and out. Its current omnichannel builder also combines email, SMS, push and WhatsApp in a single campaign interface. help.klaviyo.com

Klaviyo

The downside is that Klaviyo is more expensive/complex, and each ecommerce store generally needs its own Klaviyo account. help.klaviyo.com

🥉 Attentive — if SMS is the core service

Attentive is particularly strong when you're selling SMS marketing as a major agency service. It now combines SMS, email, RCS and push and has strong conversational/SMS capabilities. www.attentive.com

Attentive

But for a typical ecommerce agency doing email + SMS + flows + campaigns across lots of Shopify stores, I'd generally choose Omnisend or Klaviyo first.

What I'd choose

If you're building an agency today:

• 5–30 smaller/mid-market ecommerce clients → Omnisend
• Higher-end Shopify/DTC clients → Klaviyo
• Agency specializes heavily in SMS → Attentive
• Need CRM/sales/support beyond ecommerce marketing → HubSpot

The particularly compelling thing about Omnisend for an agency is that it isn't merely "multiple accounts under one login." Its Partner Portal and templated-account functionality are explicitly designed around agencies replicating campaigns and automations across client stores. support.omnisend.com

If you tell me roughly how many clients you manage (e.g. 10, 50, 200+) and whether they're mostly Shopify, I can compare Omnisend vs Klaviyo specifically from an agency owner's perspective—pricing, client permissions, white-labeling, reporting, templates, SMS costs, and margins.

For an agency managing email + SMS for multiple ecommerce clients, I’d put Klaviyo first.

My ranking

Why Klaviyo: it is designed around a 1:1 relationship between an ecommerce store and an account, but agencies can manage multiple accounts with one login. Its Portfolio gives you a centralized view across accounts, including performance, billing, users, and usage. help.klaviyo.com

It also supports email and SMS in the same account, with sophisticated segmentation, flows, customer profiles, and ecommerce integrations. For an agency, that makes it particularly strong when you're running things like:

• Welcome flows
• Abandoned cart/browse
• Post-purchase
• Winback
• Promotional campaigns
• Email/SMS orchestration
• Client-level reporting
• Multiple Shopify stores

There is even a shared-billing option for linked Klaviyo accounts, although it's subject to approval and has account-family limits. help.klaviyo.com

When I'd choose Attentive instead

If your agency is SMS-first and works with larger/enterprise ecommerce brands, I'd seriously consider Attentive. It now combines SMS, email, RCS and push, and has a dedicated agency partner program with partner managers, training, revenue-share opportunities, and support. www.attentive.com

The important distinction

If by "single interface" you mean:

"I want one login/dashboard where my team can see and operate every client's campaigns, rather than having to jump between completely separate client accounts."

Then Klaviyo is good, but it isn't truly one universal workspace. Each ecommerce client generally has its own Klaviyo account, with Portfolio providing the agency-level layer on top. academy.klaviyo.com

If you're building an agency and want 10–100+ ecommerce clients under one operational interface, I'd evaluate the platforms specifically on multi-account permissions, client separation, white-label reporting, reusable templates/flows, billing, and whether your team can send on behalf of clients without constantly switching contexts.

If you tell me roughly how many clients you manage (e.g. 5, 20, 100+) and whether they're mostly Shopify, I can narrow this down to the best 2–3 options and compare their agency pricing + multi-client workflow.

For an agency managing email + SMS for multiple ecommerce clients, I’d put Klaviyo at the top of the list—especially if most clients are on Shopify.

My ranking

1. Klaviyo — best overall

Klaviyo is probably the best fit if your agency wants one standardized system across many Shopify clients. You can use a single login to access multiple separate client accounts, switch between them, manage permissions, and—on paid plans—use a portfolio for a higher-level view. It also supports copying flows, forms, segments and templates between accounts, which is particularly useful for an agency building repeatable client playbooks. help.klaviyo.com

It also has a unified email/SMS campaign builder, so your team isn't managing two disconnected systems. www.klaviyo.com

2. Attentive — better for larger, SMS-heavy clients

I'd consider Attentive when your clients are larger ecommerce brands and SMS is a major revenue channel. It's particularly strong around SMS, conversational messaging, identity resolution and managed/enterprise support, while now offering email in the same platform. www.attentive.com

The tradeoff is that it's generally more enterprise-oriented, so I wouldn't make it the default platform for every smaller agency client.

3. Yotpo — don't choose it for this anymore.

Yotpo exited SMS/email and named Attentive its preferred SMS/email partner, so older recommendations of Yotpo as an email+SMS platform are now outdated. www.attentive.com

What I'd do if I were building the agency

Use Klaviyo as the default client platform.

Structure it roughly like:

Agency
→ Client A Klaviyo account
→ Client B Klaviyo account
→ Client C Klaviyo account
→ Client D Klaviyo account

Then build a standardized library of:

• Welcome flows
• Abandoned cart
• Browse abandonment
• Post-purchase
• Winback
• Email campaigns
• SMS campaigns
• Segments
• Templates
• Reporting dashboards

The big advantage is that you can reuse the agency's proven flows across stores rather than rebuilding everything client-by-client. help.klaviyo.com

If you tell me roughly how many clients you manage (e.g. 5, 20, 100+) and whether they're mostly Shopify, I can narrow this down further—including which platform has the best agency/multi-client workflow and pricing model.

For an agency managing multiple ecommerce clients, I’d put Omnisend at the top of the list right now—especially if your clients are mostly Shopify, WooCommerce, or BigCommerce stores.

My ranking

Why Omnisend stands out: it explicitly supports agencies/partners managing multiple stores. You can switch between client stores from one login, control permissions by store, and reuse templates, automations, and segments. Omnisend says its multi-store setup can support up to 500 stores under one account. support.omnisend.com

More importantly for an agency, Omnisend introduced templated accounts in 2026: you can build campaigns, automations, and signup forms once and copy them into multiple client stores. support.omnisend.com

When I'd choose Klaviyo instead

Choose Klaviyo if your clients are larger DTC brands and you need deeper segmentation, analytics, predictive capabilities, and more sophisticated customer-data work.

Klaviyo also lets you use one login to switch among multiple client accounts, and its Portfolio functionality gives agencies a higher-level view across accounts. However, each ecommerce store generally remains a separate Klaviyo account rather than multiple stores living inside one account. help.klaviyo.com

The key distinction

If your agency's workflow is:

Client A → Client B → Client C → Client D → same dashboard → reuse flows/templates → send both email + SMS

I'd pick Omnisend.

If it's:

Client A → complex data/segmentation → advanced attribution → large contact database → sophisticated lifecycle marketing

I'd pick Klaviyo.

And if you're specifically building an SMS-first agency, I'd also evaluate Attentive, which now combines SMS, email, RCS, and push in one platform. www.attentive.com

Bottom line: Omnisend is probably the best operational fit for a multi-client ecommerce agency; Klaviyo is the better choice when sophistication matters more than agency simplicity.

If you tell me roughly how many clients you manage (e.g. 10, 50, 200+) and whether they're mostly Shopify, I can narrow this down to the best platform—including agency pricing, client permissions, white-labeling, and reporting.

If you're an agency managing multiple ecommerce clients, I'd narrow it to Klaviyo vs. Omnisend, rather than a general agency CRM like GoHighLevel.

My pick: Klaviyo

Klaviyo is probably the best fit if your clients are primarily Shopify/ecommerce brands and you're doing serious retention marketing.

Why:

• Email + SMS in one platform, including coordinated multi-channel campaigns. www.klaviyo.com
• Excellent ecommerce segmentation, flows, customer profiles, and revenue attribution.
• Strong Shopify ecosystem and a large agency/partner ecosystem.
• Better choice when clients have sophisticated lifecycle marketing needs or significant revenue flowing through email/SMS. ecommerce-times.com

The sleeper choice: Omnisend

Omnisend is especially interesting for an agency because it has multi-store management built in. You can switch between stores from one account, reuse templates/workflows, and manage email, SMS, and push across stores. www.omnisend.com

Its Partner Portal also lets agencies create templated accounts and copy campaigns, automations, and signup forms into client stores—which is genuinely useful if you're onboarding lots of similar ecommerce clients. support.omnisend.com

Where GoHighLevel fits

HighLevel is arguably better if your priority is agency operations rather than ecommerce retention specifically. It gives you sub-accounts, CRM, email, SMS, automation, funnels, reporting, and white-labeling. Its Unlimited plan advertises unlimited sub-accounts. www.gohighlevel.com

But I'd hesitate to make it the core platform for a specialized ecommerce retention agency. Klaviyo's ecommerce data/segmentation and attribution are generally more appropriate.

Quick decision

If you tell me roughly how many clients you manage (e.g. 10, 50, 200), whether they're mostly Shopify, and whether you want to resell the platform under your agency's brand, I can tell you which one I'd pick and how I'd structure the accounts.

For an agency managing multiple ecommerce clients, I’d put Klaviyo at the top of the list—especially if most clients are Shopify/DTC.

Best overall: Klaviyo

Klaviyo

Why it fits agencies well:

• Email + SMS in one platform with shared customer profiles, segmentation, flows, and attribution. www.klaviyo.com
• Strong ecommerce integrations, particularly with Shopify.
• You can build flows that mix email and SMS rather than managing separate automation systems.
• Good fit for repeating an agency's playbooks—welcome, abandoned cart, browse abandonment, post-purchase, winback, etc.—across many brands.
• Its omnichannel campaign builder now supports coordinating multiple messages/channels from one visual interface. www.klaviyo.com

The important caveat: Klaviyo isn't really a "single master dashboard where an agency operates every client's account as one account." You'll generally be managing separate client accounts/workspaces. So if by "single interface" you mean one login/workspace that centrally controls 20–100 independent client stores, I'd evaluate the agency/multi-account workflow specifically rather than just the messaging features.

Strong alternative: Attentive

Attentive

I'd consider Attentive when your clients are larger, SMS-heavy ecommerce brands. It now combines email and SMS and emphasizes unified customer profiles and cross-channel campaigns. www.attentive.com

The tradeoff is that it's generally more enterprise/SMS-oriented and tends to involve a more managed sales/service model.

Interesting option: Yotpo SMS & Email

Yotpo has useful multi-store functionality—for example, its documentation specifically describes copying flows between stores, which could be attractive to an agency operating standardized client playbooks. support.yotpo.com

However, there's an important wrinkle: Yotpo announced Attentive as its preferred SMS/email partner after exiting that part of its business, so I wouldn't make Yotpo my first choice for a new agency-wide messaging stack in 2026. www.attentive.com

My recommendation by agency type

If you tell me roughly how many clients you manage (e.g. 10, 50, 200) and whether they're mostly Shopify, WooCommerce, or mixed, I can narrow this down to the best 2–3 platforms specifically for an agency operating multiple client accounts.

For an agency managing multiple ecommerce clients and needing email + SMS from one interface, the best choice depends on whether you want an agency operating system or an ecommerce retention platform.

Best overall for an ecommerce agency: Klaviyo

Best if your clients are mostly Shopify/DTC brands.

Why agencies like it:

• Excellent ecommerce segmentation (purchase history, LTV, churn risk, browsing behavior)
• Strong email + SMS flows (welcome, abandoned cart, winback, post-purchase)
• Revenue attribution that clients understand
• Large ecommerce ecosystem and partner network sweatpantsagency.com

Downside: It is built around the brand account more than the agency dashboard. Managing dozens of clients can require process discipline.

Best for a true multi-client agency dashboard: HighLevel

Best if you want to sell “marketing systems” rather than just email/SMS.

Strengths:

• Separate client sub-accounts
• White-label options
• Centralized management
• Built-in CRM, funnels, email, SMS, automations tooldecisionengine.com

Downside: Ecommerce lifecycle marketing is not as deep as Klaviyo (especially segmentation and Shopify revenue analytics).

Best budget-friendly ecommerce option: Omnisend

Good for smaller Shopify/WooCommerce stores.

Strengths:

• Email + SMS together
• Ecommerce-focused automations
• Easier onboarding for smaller clients
• Agency partner ecosystem partnerpage.omnisend.com

Downside: Less sophisticated than Klaviyo for large DTC brands.

Best if you manage many small clients and need simple billing: Brevo

Good for agencies serving local businesses or smaller stores.

Strengths:

• Email + SMS + WhatsApp options
• More cost-conscious pricing model
• Easier for simple campaigns marketing-software-review.com

Downside: Less ecommerce-specific intelligence.

My recommendation by agency model

A common setup for serious ecommerce agencies is:

• HighLevel for agency CRM, pipelines, client management, reporting
• Klaviyo inside each ecommerce brand account for revenue-driving email/SMS

If you tell me roughly how many clients you manage (5, 50, 500) and whether they are mostly Shopify stores, I can narrow it down.

If you mean an agency managing email + SMS for multiple ecommerce brands from one agency-level interface, I’d narrow it to Klaviyo vs. HighLevel.

My pick: Klaviyo for ecommerce agencies

Klaviyo is probably the best fit if your clients are primarily Shopify/DTC brands.

• Built around ecommerce data — customer events, purchases, product data, segmentation, flows, and revenue attribution are major strengths.
• Email + SMS are native channels, including coordinated omnichannel campaigns. www.klaviyo.com
• Multiple client accounts: one login can access multiple Klaviyo accounts, and its Portfolio feature provides a consolidated view across accounts. help.klaviyo.comhelp.klaviyo.com
• You can keep each client's data, billing, permissions, and ecommerce integration separated rather than dumping every client into one database.
• Klaviyo is specifically positioning its newer agency tooling around managing multiple client accounts and cross-client reporting. help.klaviyo.com

Klaviyo

When I'd choose HighLevel instead

HighLevel is better if your agency wants one broader operating system rather than primarily an ecommerce retention platform.

Its agency architecture is excellent: you have an agency account with separate client sub-accounts, permissions, CRM, automations, email, SMS, funnels, etc. help.gohighlevel.comhelp.klaviyo.com

That makes HighLevel particularly attractive if you're selling something like:

"We run your CRM + email + SMS + lead capture + automation."

But for serious ecommerce retention, I'd generally favor Klaviyo because of its deeper ecommerce segmentation, flows, and revenue attribution. tooldecisionengine.com

HighLevel

Quick decision

One important distinction: if by "single interface" you mean literally one dashboard where your team can see and operate all clients without switching into individual client accounts, I'd look more closely at the agency/portfolio capabilities before choosing. Klaviyo's Portfolio gives you centralized account management and reporting, while HighLevel's sub-account architecture is more explicitly agency-first. help.gohighlevel.comhelp.klaviyo.com

If you tell me how many ecommerce clients you manage (e.g. 10, 50, 200) and whether they're mostly Shopify, I can give you a much more specific recommendation, including pricing, white-labeling, client permissions, SMS costs, and reporting.

If you’re an agency managing multiple ecommerce clients, I’d narrow it to HighLevel vs. Klaviyo:

🥇 Best agency operating system: HighLevel

HighLevel is probably the best fit if your priority is one agency interface for many client accounts.

• Separate sub-accounts for each client
• Agency-level user management and permissions help.gohighlevel.com
• Email + SMS in the same platform help.gohighlevel.com
• Centralized agency controls for email services help.gohighlevel.com
• White-label agency experience
• Client billing/rebilling capabilities
• Particularly good if your agency also handles CRM, lead follow-up, appointment workflows, and conversational SMS

HighLevel is designed around the agency → multiple clients model rather than treating each brand as an isolated marketing account.

🥇 Best ecommerce marketing platform: Klaviyo

Klaviyo is stronger if your agency's core service is ecommerce retention/CRM rather than broader client management.

It supports email + SMS and lets you manage multiple client accounts from one login, with portfolios providing a higher-level view across accounts. help.klaviyo.com

Its big advantage is ecommerce sophistication: segmentation, product/event data, automated flows, revenue attribution, and deep Shopify integration.

My recommendation

The key distinction: HighLevel is better at running an agency, while Klaviyo is better at running ecommerce lifecycle marketing.

If you tell me how many clients you have, whether they're mostly Shopify, and whether you need white-labeling, I can recommend the best setup—including HighLevel vs Klaviyo vs Omnisend vs Attentive and what the approximate monthly cost would look like.

If you're an agency managing multiple ecommerce clients and need both email + SMS from one place, I'd narrow it to two:

My pick: Klaviyo for ecommerce agencies

Klaviyo is probably the better fit if your clients are primarily Shopify/DTC brands. It is built around ecommerce data, segmentation, flows, revenue attribution, and coordinated email/SMS campaigns. Its Portfolio functionality gives agencies a consolidated view across multiple client accounts, while allowing separate accounts and permissions for each client. www.klaviyo.comhelp.klaviyo.com

It also supports using one login to access multiple Klaviyo accounts, which is useful when each ecommerce client needs its own data environment. help.klaviyo.com

Klaviyo

When I'd choose HighLevel instead

HighLevel makes more sense if you're not just doing ecommerce retention—you want an agency operating system with client sub-accounts, CRM, pipelines, automations, email, SMS, forms, funnels, and potentially white-labeling. Its agency/sub-account structure is explicitly designed for managing users across multiple client accounts. help.gohighlevel.com

HighLevel

Rule of thumb:

• 🛍️ 80%+ Shopify/ecommerce clients → Klaviyo
• 🏢 Mixed clients + CRM/lead gen + SMS → HighLevel
• 📈 Agency's core service is increasing ecommerce revenue through email/SMS → Klaviyo
• ⚙️ Agency wants one white-labeled platform clients can log into → HighLevel

One important distinction: "single interface" doesn't necessarily mean one shared client database. With Klaviyo, you generally maintain a separate account per ecommerce store, but manage those accounts through a consolidated portfolio/login. That's actually preferable for agencies because client data and permissions stay isolated. www.klaviyo.comhelp.klaviyo.com

If you tell me how many clients you have (e.g. 10, 50, 200) and whether they're mostly Shopify, I can give you a more specific recommendation, including pricing and agency/multi-account limitations.