ChatGPT answers, unedited

For a SaaS/product with monthly and annual plans, automated proration, and mid-cycle upgrades, I’d shortlist these:

1. chargebee.com — best overall for subscription-heavy SaaS

This would be my first choice if billing logic is a core part of your product.

• Supports monthly and yearly billing.
• Automatically handles upgrades, downgrades, and mid-cycle proration. Chargebee Chargebee
• Lets you choose day-based or millisecond-based proration. Chargebee
• Can apply credits/charges automatically and generate adjusted invoices.
• Includes hosted checkout, customer self-service portals, coupons, invoicing, and subscription management. Chargebee
• Particularly strong if you expect to add usage-based, seat-based, or hybrid pricing later.

Best for: SaaS companies that want a dedicated subscription-billing engine without building the billing state machine themselves.

2. stripe.com — best if you're already using Stripe

Stripe is probably the simplest choice if your payments already run through Stripe.

• Supports recurring monthly and annual subscriptions.
• Automatically handles proration when plans are upgraded/downgraded. Stripe
• Supports scheduled subscription changes and more sophisticated pricing models. Stripe
• Has strong APIs and a large ecosystem.
• Current pay-as-you-go Billing pricing is 0.7% of Billing volume, excluding one-off invoices. Stripe

Best for: Developer-led companies that want maximum control and don't need Chargebee's more extensive subscription-management layer.

3. paddle.com — best if you want Merchant of Record

Paddle is compelling if you want to sell internationally without taking on as much tax/compliance infrastructure yourself.

It supports recurring monthly/annual billing, proration, upgrades and downgrades, while acting as the Merchant of Record and handling payment processing and global tax compliance. Paddle

Best for: B2C/B2B SaaS selling globally where tax/VAT/GST compliance is a major concern.

My pick

If all you need is: $20/month → $200/year → upgrade mid-cycle → automatically credit unused time and charge the difference, Stripe Billing is probably enough. If you expect complicated plans, seats, add-ons, usage billing, contracts, or finance/revenue-recognition requirements, I'd lean Chargebee. Chargebee Chargebee

If you tell me your stack (e.g. Stripe/React/Node), number of customers, and whether this is B2B or B2C, I can recommend one and outline exactly how I'd implement the monthly/annual upgrade + proration flow.

If your core requirement is monthly + annual plans with automatic proration and self-serve upgrades, I’d shortlist these three:

1. stripe.com — best default

• Supports monthly and annual recurring prices.
• Automatically prorates upgrades, downgrades, cancellations, and pauses. Stripe Stripe
• Strong APIs and checkout components, so it's a good fit if you have engineering resources.
• Current pay-as-you-go Billing pricing is 0.7% of Billing volume, excluding one-off invoices. Stripe
• Best if you want maximum control over your billing logic and payment stack.
• paddle.com — best for SaaS wanting less billing infrastructure

• Handles subscriptions, upgrades/downgrades, proration, payment collection, and customer management.
• Proration is calculated automatically, down to the minute. Paddle Paddle Developer Docs
• Particularly attractive if you sell internationally because Paddle operates as a Merchant of Record, handling payments, tax compliance, and related responsibilities. Paddle
• Good choice if you don't want to build as much billing/tax infrastructure yourself.
• chargebee.com — best for sophisticated subscription businesses

• Very strong subscription lifecycle management.
• Supports automated upgrades/downgrades and proration, including configurable day-based or millisecond-based calculations. Chargebee Chargebee
• Better suited than Stripe if your billing eventually gets complicated—multiple plans, add-ons, usage billing, complex invoicing, etc.
• Current pricing starts at 0.80% of monthly billing value on its pay-as-you-go Flow plan, with a $0 platform fee. Chargebee

My pick

For a typical SaaS with something like:

Starter — $20/month or $200/year
Pro — $50/month or $500/year
Business — $100/month or $1,000/year

I'd choose Stripe Billing unless you specifically want Merchant-of-Record handling, in which case I'd choose Paddle.

The key thing to verify during implementation is how you want monthly → annual upgrades handled. For example, if someone has 15 days left on a $50/month plan and switches to a $500/year plan, you can have the platform credit the unused portion and charge the appropriate difference immediately. Paddle explicitly supports this billing-frequency change and proration behavior. Paddle Developer Docs

If you tell me your stack (e.g. Next.js, Django, Rails), number of plans, and whether you need tax/VAT handled, I can recommend the best one and show you exactly how I'd structure the billing.

If your core requirement is monthly + annual subscriptions with automatic mid-cycle upgrades/downgrades and proration, I’d shortlist these:

1. stripe.com — my default recommendation

Stripe supports recurring subscriptions, both monthly and annual pricing, and built-in prorations when customers change plans. It also supports scheduled subscription changes, a customer portal, trials, coupons, and more complex pricing models. Stripe Stripe

For example:

Customer is on $20/month → upgrades halfway through the month to $50/month → Stripe credits the unused portion of the $20 plan and charges the appropriate amount for the $50 plan.

Choose Stripe if: you're building the product yourself and want maximum API/control without adopting a heavyweight billing platform.

2. paddle.com — best if you want billing infrastructure outsourced

Paddle explicitly handles subscription proration automatically and supports moving customers between plans through its dashboard or API. It also bundles payments, subscription management, tax/compliance, and a customer portal. Paddle Paddle

Choose Paddle if: you're a SaaS/digital-product business and would rather have Paddle handle more of the payment/tax/billing complexity.

3. chargebee.com — best for complex subscription logic

Chargebee is particularly strong when billing gets complicated. It supports automatic upgrades/downgrades and proration, with configurable day-based or millisecond-based calculations. It can also schedule changes for a future date and automatically generate the resulting credits/charges. Chargebee Chargebee

Choose Chargebee if: you expect multiple plans, add-ons, seats, usage billing, complex invoicing, or sophisticated finance requirements.

My recommendation

For a typical SaaS product, I'd start with Stripe Billing.

A clean setup would be:

• Product: Your SaaS
• Prices: Monthly and Annual
• Subscription: Customer's current plan + billing interval
• Upgrade: Immediate with automatic proration
• Downgrade: Either immediate with credit or scheduled for next renewal
• Billing portal: Customer manages payment method and subscription
• Webhooks: Your app listens for subscription/payment changes
• Annual → monthly / monthly → annual: Handle as a subscription price change with your chosen proration policy

If you tell me what you're selling (SaaS, membership, API, course, etc.) and roughly how many customers you expect, I can recommend Stripe vs Paddle vs Chargebee specifically for your situation, including the likely pricing and implementation complexity.

For example:

Customer is on $20/month → upgrades halfway through the month to $50/month → Stripe credits the unused portion of the $20 plan and charges the appropriate amount for the $50 plan.

Choose Stripe if: you're building the product yourself and want maximum API/control without adopting a heavyweight billing platform.

2. paddle.com — best if you want billing infrastructure outsourced

Paddle explicitly handles subscription proration automatically and supports moving customers between plans through its dashboard or API. It also bundles payments, subscription management, tax/compliance, and a customer portal. paddle.com paddle.com

Choose Paddle if: you're a SaaS/digital-product business and would rather have Paddle handle more of the payment/tax/billing complexity.

If your core requirement is monthly + annual subscriptions with automatic proration and upgrades, I’d shortlist these:

1. Stripe Billing — best default for most SaaS

Stripe supports monthly and annual recurring prices, customer self-service, plan changes, and automatic prorations for upgrades/downgrades. It also supports more advanced pricing models if you later add seats, usage, tiers, or hybrid pricing. stripe.com

Why I’d pick it: easiest ecosystem and developer experience, especially if you're already using Stripe Payments.

Example:
Customer pays $100/month → upgrades to $200 on day 15 → Stripe credits the unused portion of the $100 plan and charges for the remaining portion of the $200 plan.

2. Chargebee — best if billing complexity will grow

Chargebee is particularly strong for subscription management. It supports annual/monthly billing, automated upgrades/downgrades, configurable proration, self-service portals, invoices, coupons, and more sophisticated billing workflows. www.chargebee.com

It gives you particularly fine-grained control over proration: day-based or millisecond-based calculations, and the ability to apply changes immediately or at renewal. www.chargebee.com

Why I’d pick it: if you're building a serious B2B SaaS where billing logic, invoicing, credits, contracts, and finance workflows will become important.

3. Paddle Billing — best if you want Merchant of Record

Paddle handles subscription billing, upgrades/downgrades, proration, payment collection, tax compliance, and revenue-related operations as a Merchant of Record. www.paddle.com

Why I’d pick it: particularly attractive if you sell SaaS internationally and don't want to build your own global tax/compliance infrastructure.

My recommendation

For a new SaaS, I'd start with Stripe Billing unless you specifically need Chargebee's deeper billing operations or Paddle's Merchant-of-Record model.

One important thing to verify before committing: how you want annual ↔ monthly upgrades to behave. For example, if someone is on $100/month and switches to $1,000/year halfway through the month, you need to decide whether to credit the unused month, charge the net difference immediately, preserve the original renewal date, or move them to an annual anniversary. Stripe and Chargebee can support these kinds of workflows, but the exact implementation matters. www.chargebee.com

If you tell me your stack (e.g. Next.js + Stripe, Laravel, Django) and your pricing structure, I can recommend the simplest setup and explain exactly how I'd implement the proration/upgrade flow.

For your requirements, I’d shortlist Paddle, Stripe Billing, and Chargebee. My default recommendation would be Paddle if you’re selling SaaS/digital products directly to customers.

🥇 My pick: Paddle

Paddle Billing

Paddle handles subscriptions, payments, invoicing, tax compliance, and subscription lifecycle management in one system. It explicitly supports automatic proration and upgrades/downgrades, including switching between monthly and annual billing. www.paddle.comdeveloper.paddle.com

Its proration engine is particularly nice: it calculates changes to the minute and lets you choose whether the prorated amount is charged immediately, charged at the next renewal, or not charged. developer.paddle.com

For example:

Customer pays $30/month → upgrades to $60/month halfway through the month → Paddle credits the unused portion of $30 and charges the appropriate portion of the $60 plan.

You can also switch a customer from monthly → annual and control whether that change generates a prorated charge. www.paddle.comdeveloper.paddle.com

When I'd choose Stripe instead

Stripe Billing

Choose Stripe if you want maximum developer control or you're already heavily invested in the Stripe ecosystem. It's excellent billing infrastructure, but you'll generally have more responsibility around the overall payments/tax/compliance stack than with Paddle.

When I'd choose Chargebee

Chargebee

Chargebee is especially compelling if you expect complicated pricing: seat-based + usage-based billing, multiple frequencies, coupons, add-ons, enterprise contracts, etc. Its proration engine supports day- or millisecond-level calculations and automatically generates credits/charges for plan changes. www.chargebee.com

Bottom line:

• Simple/medium SaaS → Paddle
• Developer-first/custom billing → Stripe Billing
• Complex B2B SaaS billing → Chargebee

If you tell me your plan prices, whether it's B2B or B2C, and whether you need a hosted checkout/customer portal, I can recommend the best one and outline exactly how I'd implement monthly → annual upgrades and proration.

If your main requirement is monthly + annual plans with automatic proration when customers upgrade/downgrade, my top recommendation is Chargebee.

My picks

1. Chargebee — my recommendation
Chargebee is particularly strong if subscription management is a core part of your product. It automatically calculates credits for unused time on the old plan and charges for the remaining time on the new plan. It supports both day- and millisecond-based proration, and can handle changes between monthly and longer billing terms. www.chargebee.comwww.chargebee.com

It also supports immediate upgrades, changes at renewal, scheduled changes, self-service subscription management, dunning, and more. www.chargebee.com

Chargebee

2. Stripe Billing — best if you're already on Stripe
Stripe Billing is probably the simplest choice if your payments infrastructure is already Stripe. It supports recurring subscriptions, subscription schedules, and mid-cycle billing changes. Its current pay-as-you-go Billing price is 0.7% of Billing volume, on top of applicable payment processing fees. stripe.com

Stripe Billing

3. Paddle — best if you want less billing infrastructure to manage
Paddle combines recurring billing with payments, tax/compliance, dunning, proration, and upgrades/downgrades. That can be attractive if you don't want to assemble Stripe + tax + subscription-management pieces yourself. www.paddle.com

Paddle Billing

What I'd choose

• Building a SaaS and want sophisticated subscription logic: Chargebee
• Already using Stripe and want a developer-friendly solution: Stripe Billing
• Selling globally and want payments/tax/billing bundled together: Paddle
• Large/mature subscription operation: Recurly

One important thing to test before committing: monthly → annual upgrades. You want the platform to correctly credit the unused monthly period and calculate the new annual charge, while preserving or intentionally changing the renewal date. Chargebee explicitly supports term changes and prorated billing for this scenario. www.chargebee.comwww.chargebee.com

If you tell me your stack (e.g. Next.js, Django, Laravel), number of customers, and whether you need Stripe or tax/VAT handling, I can narrow this to one and compare the implementation effort and pricing.

If your main requirement is monthly + annual subscriptions with automatic proration when customers upgrade/downgrade, I’d recommend Paddle.

My pick: Paddle

Paddle is particularly well suited to SaaS because it handles the subscription lifecycle for you:

• Monthly and annual plans
• Automatic proration for mid-cycle changes, calculated to the minute developer.paddle.comdeveloper.paddle.com
• Plan upgrades/downgrades, including Pro → Enterprise, etc. developer.paddle.comdeveloper.paddle.comwww.paddle.com
• Monthly → annual and annual → monthly billing changes with configurable proration behavior developer.paddle.comdeveloper.paddle.comwww.paddle.com
• Add-ons, seats, and quantities, with proration when they change developer.paddle.com
• Customer self-service through its customer portal
• Payments, invoicing, tax compliance, and subscription management in one system; Paddle acts as Merchant of Record, which can substantially reduce your tax/compliance work. www.paddle.com

One especially nice feature is that you can preview a subscription change before applying it, so your UI can show something like:

Current plan: $50/mo
Remaining credit: −$20
New plan: $100/mo
Due today: $80

Paddle's API supports several proration behaviors, including charging the prorated amount immediately, charging it at the next renewal, or not billing the change at all. developer.paddle.comdeveloper.paddle.com

The main alternative: Stripe Billing

I'd choose Stripe Billing instead if you want maximum control over the billing/payment infrastructure or you're already deeply invested in Stripe. It supports sophisticated subscription schedules and billing models, but you'll generally own more of the implementation around your billing experience and tax/compliance stack. Stripe's current Billing pricing starts at 0.7% of Billing volume on its pay-as-you-go option. stripe.com

Quick decision

Bottom line: For a typical SaaS product with Starter/Pro/Enterprise × monthly/annual, I'd start with Paddle. Its subscription model maps very cleanly to that use case, and you won't need to build your own proration engine. developer.paddle.comdeveloper.paddle.comwww.paddle.com

For selling monthly and annual plans with automated proration, upgrades, and downgrades, I’d shortlist these platforms:

My recommendation by scenario

1. Simple SaaS (Starter → Pro → Business tiers)
→ Stripe Billing

Example:

• Monthly: $29/mo
• Annual: $290/year
• User upgrades from Starter to Pro mid-month
• Stripe calculates credit for unused Starter time and charges the difference

Good if you already use Stripe payments and have engineering resources.

2. You want the least billing infrastructure work
→ Paddle

Good if you want:

• subscriptions
• invoices
• tax handling
• customer self-service
• global payments
• proration rules

Paddle supports mid-cycle changes with configurable proration behavior (for example, charge immediately, charge at next renewal, or skip billing). developer.paddle.com

3. B2B SaaS with seats, add-ons, contracts, or complex plans
→ Chargebee

Better when you need:

• annual contracts
• multiple subscription components
• usage-based add-ons
• enterprise billing operations

Chargebee supports automatic handling of upgrades, downgrades, renewals, and prorated changes. www.paddle.comwww.chargebee.com

Features I’d require before choosing

Make sure the platform supports:

• ✅ Monthly ↔ annual plan switching
• ✅ Immediate upgrades with prorated charges
• ✅ Downgrades with credits applied later
• ✅ Customer self-service billing portal
• ✅ Coupons and promotions
• ✅ Failed payment retries (dunning)
• ✅ Webhooks for subscription state changes
• ✅ Seat/add-on changes if you expect growth

For most new SaaS products, I’d start with Stripe Billing unless you specifically want merchant-of-record tax handling (choose Paddle) or expect complex B2B pricing (choose Chargebee).

If your core requirement is monthly + annual plans, automatic proration, and seamless upgrades/downgrades, I’d shortlist these:

1. Paddle — best all-in-one choice

Paddle is particularly well suited if you’re selling SaaS or digital subscriptions. It supports monthly/annual billing, plan changes, and automatic proration down to the minute. You can also control whether the prorated amount is charged immediately, billed at renewal, or not billed. www.paddle.comdeveloper.paddle.com

Why I’d pick it: less billing infrastructure for you to build, with checkout, payments, subscription management, and customer self-service in one platform.

2. Stripe Billing — best if you want maximum flexibility

Stripe is excellent if you already use Stripe Payments or want to build a highly customized billing experience. It supports subscription schedules and automated billing workflows, though you’ll generally be doing more implementation yourself than with Paddle. stripe.com

Why pick it: strongest ecosystem and developer flexibility.

3. Chargebee — best for sophisticated SaaS billing

Chargebee is a strong choice if you expect complex pricing: multiple plans, add-ons, usage billing, seats, discounts, or enterprise contracts. It explicitly handles mid-cycle upgrades/downgrades and prorated credits/charges, with configurable billing precision. www.chargebee.com

Why pick it: more powerful subscription-management functionality, especially as pricing gets complicated.

My recommendation

For your exact requirement, I'd start with Paddle. Its monthly → annual transition and plan upgrades are first-class subscription operations, and its proration engine handles the messy calculation automatically. www.paddle.comdeveloper.paddle.com

If you tell me what you're selling (SaaS, membership, services, etc.) and roughly how many customers you expect, I can narrow this down based on pricing, checkout, taxes, and implementation effort.

For your requirements—monthly + annual plans, automatic proration, and seamless upgrades/downgrades—I’d shortlist these:

My recommendation: Chargebee

I'd pick Chargebee if billing logic is an important part of your product. It supports recurring plans, upgrades/downgrades, and automatic mid-cycle proration, with configurable day-based or millisecond-based calculations. www.chargebee.comwww.chargebee.comdeveloper.paddle.com

For example, if someone is paying $30/month and upgrades halfway through the month to $100/month, Chargebee can automatically credit the unused portion of the $30 plan and charge the appropriate portion of the $100 plan. www.chargebee.com

It also supports more sophisticated models if you eventually add seats, usage-based billing, add-ons, multiple billing frequencies, or custom contracts. www.chargebee.com

When I'd choose Paddle instead

If you want the least billing infrastructure to build yourself, Paddle is particularly attractive. It handles subscription payments, proration, upgrades/downgrades, failed-payment recovery, and related billing operations. www.paddle.com

Its API specifically supports switching monthly → annual and controlling whether the resulting proration is charged immediately, at the next renewal, or not billed. developer.paddle.com

Bottom line:

• Chargebee → best if you expect billing to get sophisticated.
• Paddle → best if you want a more turnkey SaaS billing/payment stack.
• Stripe Billing → best if your existing infrastructure is already built around Stripe; its current Billing pricing is usage-based starting at 0.7% of Billing volume on the pay-as-you-go option. stripe.com

If you tell me what you're selling (SaaS, memberships, courses, etc.), your expected number of customers, and whether you already use Stripe, I can narrow this to one platform and explain exactly how I'd implement the monthly/annual upgrade flow.

If your core requirement is monthly + annual subscriptions with automatic mid-cycle upgrades/downgrades and proration, I’d shortlist these:

My recommendation: Paddle

For a straightforward SaaS with, say, Basic $20/month or $200/year and Pro $50/month or $500/year, Paddle handles the billing mechanics for you. It supports recurring subscriptions, plan changes, and automatic proration. www.paddle.com

Importantly, Paddle can prorate upgrades, downgrades, quantity changes, and billing-frequency changes, and its proration engine calculates to the minute. You can also choose whether the prorated amount is charged immediately, charged at the next renewal, or not billed. developer.paddle.com

So a customer could:

Monthly Basic → Monthly Pro

• Credit unused Basic time
• Charge remaining Pro time
• Collect the net difference automatically

Or:

Monthly Pro → Annual Pro

• Replace the monthly price with the annual price
• Apply whatever proration behavior you configure

When I'd choose Stripe instead

If you're already using Stripe for payments, Stripe Billing is probably the most natural choice. Its current Billing pricing is usage-based, starting at 0.7% of Billing volume on the pay-as-you-go plan. stripe.com

The main advantage is ecosystem flexibility: you get Stripe Checkout, Customer Portal, Payment Links, Billing APIs, webhooks, etc., rather than introducing another payment platform.

When I'd choose Chargebee

Choose Chargebee if you expect billing to become significantly more complicated—multiple pricing models, seats, usage billing, add-ons, complex credits, revenue recognition, or enterprise contracts. Chargebee explicitly supports automated mid-cycle upgrades/downgrades and proration, including configurable day- or millisecond-based calculations. www.chargebee.com

Bottom line:

• Simple SaaS → Paddle
• Already on Stripe / developer-heavy → Stripe Billing
• Complex subscription business → Chargebee

If you tell me your plan structure (prices, monthly/annual discounts, and whether upgrades should charge immediately), I can recommend the best one and show exactly how the proration would work.

If you’re selling monthly and annual SaaS plans and want upgrades/downgrades with automatic proration, my default recommendation is Stripe Billing.

Best options

Stripe Billing natively supports monthly/yearly prices and prorates subscription changes. For example, if someone upgrades halfway through a month, Stripe can credit the unused portion of the old plan and charge for the remaining portion of the new plan. You can also configure whether the prorated amount is invoiced immediately or carried to the next invoice. docs.stripe.com

Stripe's newer flexible billing mode is particularly attractive if you're building a new integration; Stripe recommends it for more predictable proration and flexible billing behavior. docs.stripe.com

Stripe Billing

When I'd choose Chargebee instead

Choose Chargebee if you expect things like:

• Multiple pricing models or add-ons
• Complex B2B billing
• Usage-based billing
• Lots of billing changes and edge cases
• More non-engineering control over pricing/billing
• Sophisticated invoicing and revenue workflows

Chargebee explicitly handles mid-cycle upgrades/downgrades and automatic proration, including day- or millisecond-based calculations. www.chargebee.com

Chargebee

When I'd choose Paddle

If you're selling software globally and don't want to build your own tax/compliance/payment infrastructure, Paddle is compelling because it operates as a Merchant of Record while also handling subscriptions, upgrades, downgrades, proration, payment collection, and tax compliance. docs.stripe.comdocs.stripe.comwww.paddle.com

Paddle

My recommendation

For a typical SaaS:

Stripe Billing → Checkout → Customer Portal → webhooks

I'd use Stripe if your requirements are basically:

Monthly plan + annual plan + upgrade/downgrade + automatic proration + recurring card payments.

It's comparatively straightforward and gives you API-level control. Stripe currently charges 0.7% of Billing volume on its pay-as-you-go Billing plan, on top of applicable payment-processing fees. stripe.com

If you tell me your stack (e.g. Next.js, Laravel, Django), number of plans, and whether customers can switch monthly ↔ annual, I can recommend the exact setup and proration rules I'd use.

For annual + monthly subscriptions with automatic proration and upgrades, I’d shortlist these three:

1. Paddle — best overall for SaaS
   • Supports monthly and annual recurring plans.
   • Handles upgrades/downgrades and billing-frequency changes.
   • Automatic proration can be billed immediately or at the next renewal.
   • Proration is calculated to the minute. www.paddle.com
   • Particularly attractive if you want payments + subscription billing + tax/compliance handled together.

2. Chargebee — best for sophisticated subscription logic
   • Strong support for annual/monthly plans, upgrades, downgrades, add-ons, usage billing, and complex pricing.
   • Handles mid-cycle changes automatically and generates prorated credits/charges.
   • Supports day-based or millisecond-based proration. www.chargebee.com
   • Better choice than Paddle if you expect complex SaaS pricing or enterprise billing.

3. Stripe Billing — best if you're already on Stripe
   • Natural choice if your payments infrastructure is already Stripe.
   • Stripe Billing supports recurring subscription billing and has usage-based billing capabilities.
   • Pricing currently starts at 0.7% of Billing volume on its pay-as-you-go plan. stripe.com
   • The tradeoff is that you'll generally build more of the subscription-management UX/workflow yourself compared with a more specialized platform.

My recommendation

For a typical SaaS product: choose Paddle.
For a more complex B2B SaaS: choose Chargebee.
If you're already deeply invested in Stripe: use Stripe Billing.

One important detail: if your desired flow is “customer is on $20/month, upgrades to $40/month halfway through the month, immediately charges only the prorated difference, and then renews at $40/month”, all three can support that general model, but Paddle and Chargebee make the subscription-management portion particularly explicit in their APIs. developer.paddle.com

If you tell me your stack (e.g. Next.js, Laravel, Django), number of customers, and whether you need Stripe Connect/tax handling, I can recommend the best one and outline the exact upgrade/proration architecture.

If your main requirement is monthly + annual plans with automatic proration when customers upgrade/downgrade, I’d shortlist these:

My recommendation: Paddle

Paddle is particularly attractive if you want to avoid building billing logic yourself. It handles subscription creation, recurring payments, upgrades/downgrades, and proration. Its proration engine calculates adjustments to the minute, and you can control whether the prorated amount is charged immediately, charged at renewal, or not billed. www.paddle.com

It also supports switching billing frequency—for example, monthly → annual—through the subscription API. developer.paddle.com

A particularly nice feature is the hosted customer portal: customers can upgrade/downgrade themselves, with Paddle handling the corresponding proration. developer.paddle.com

When I'd choose Stripe instead

Choose Stripe Billing if you already have a Stripe-based payments stack or want maximum control over the checkout and billing experience. Its current Billing pricing starts at 0.7% of Billing volume on the pay-as-you-go option. stripe.com

When I'd choose Chargebee

I'd pick Chargebee if you expect your billing model to become complicated—multiple add-ons, usage-based billing, seats, coupons, multiple billing frequencies, enterprise contracts, etc. Chargebee explicitly supports automated mid-cycle upgrades/downgrades and proration, including day- or millisecond-based calculations. www.chargebee.com

Bottom line: For a straightforward SaaS with Monthly / Annual → upgrades → automatic prorated charges, I'd start with Paddle. If you're already deeply invested in Stripe, use Stripe Billing instead.