Use Case · SaaS and Subscriptions

Subscription Billing for SaaS Companies

Trials, tiered plans, monthly and annual billing, add-ons, upgrades with proration, usage on top, and retries when a card fails. All from one catalog you control.

Kill Bill (open source): catalog, trials, add-ons, proration, dunningAviate (enterprise): catalog UI, coupons, prepaid credits

Last updated: October 2026

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Illustration with demo prices: one customer from trial to upgrade to a failed payment and its retry.

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Short answer

How do you bill SaaS subscriptions with trials, upgrades and add-ons?

With Kill Bill, you describe products, plans and prices in a catalog. A plan can start with a free trial or a discount phase, then bill monthly or annually. Add-on products attach to a base plan, and usage can be billed on top. When a customer upgrades in the middle of a period, Kill Bill credits the unused part of the old plan and charges the new one for the rest of the period. Failed payments are retried on a schedule you set, and dunning rules decide what happens next. Kill Bill is open source. Aviate, its enterprise offering, adds a catalog UI, coupons and prepaid credits.

Kill Bill and Aviate

Who Does What for SaaS Billing

Aviate runs on top of Kill Bill. With Aviate, you keep everything in the left column and add the right one.

Kill Bill

Open source, Apache 2.0. Free, installed and run by you, without support or SLA.

Catalog with phasesTrial, discount, fixed term and evergreen phases on each plan
Plans, billing periods and add-onsMonthly, annual and other periods, add-ons attached to a base plan
Plan changes with prorationImmediate or at the end of the term, set by catalog rules
Usage on top of a planUsage billed in arrears with the subscription
Catalog versions and price overridesNew prices from a date, custom prices per subscription
Payment retries, dunning, pause, resume and cancel

Aviate

The enterprise offering of Kill Bill. Licensed, with Level 3 support from the engineers.

Catalog in AviateCreate products, plans, phases and prices without editing XML
Aviate CouponsPercentage or fixed discounts, with a limit, an expiration date and eligible plans
Aviate WalletPrepaid and free credits, automatic top-off, expiration
Aviate TaxTax rates per country and per product
Level 3 supportThe engineers, on a dedicated Slack channel. First response within 1 business day for a P1

Features specific to one edition are tagged Kill Bill or Aviate. Untagged items apply to both.

At a Glance

SaaS Billing at a Glance

Catalog: Products, plans, phases and prices, in XML or, with Aviate, in a UI
Phases: TRIAL, DISCOUNT, FIXEDTERM and EVERGREEN
Products: BASE plans, ADD_ON products tied to a base, and STANDALONE products. Add-ons that a new base plan no longer allows are cancelled automatically
Billing day: Align subscriptions on the account, the bundle or each subscription, so a customer can be billed once a month
Currencies: A price per currency on each plan
Plan changes: Immediate, with proration, or at the end of the term. Catalog rules set the default, the API can override it
Preview: An invoice dry run shows what a plan change will cost before it happens
Price changes: Catalog versions with an effective date, and a separate date for existing subscriptions
Usage: Usage billed in arrears next to the recurring fee
Failed payments: Retries on a schedule you set, then dunning states that can block changes or pause the service
Promotions: Discount phases in Kill Bill. Coupons with Aviate
Payments: Payment plugins for Stripe, Adyen, Braintree, Hyperswitch, or your own
Reporting: The analytics plugin keeps a reporting database, with reports in Kaui

Pricing Models

SaaS Pricing Models and How to Set Them Up

Common SaaS pricing models, and the Kill Bill and Aviate features behind each one.

Model	Example	How to set it up	Edition
Free trial	14 days free, then $49 per month	A TRIAL phase before the EVERGREEN phase of the plan	Kill Bill
Introductory price	Half price for the first 3 months	A DISCOUNT phase with its own price and duration	Kill Bill
Tiered plans	Starter, Growth, Scale	One plan per tier, with change rules between them. Catalog	Kill Bill
Monthly or annual	Scale billed annually	A plan per billing period, billed in advance by default	Kill Bill
Add-ons	Extra storage for Growth customers	An ADD_ON product, available or included with chosen base plans	Kill Bill
Usage on top	API calls beyond the plan	A usage section in the plan, billed in arrears. Usage guide	Kill Bill
Price increase	New price for new customers first	A catalog version with an effective date, and a later date for existing subscriptions	Kill Bill
Custom deal	A discount for one large customer	Price overrides on that subscription	Kill Bill
Coupons	WELCOME20 for 20% off	Aviate Coupons: percentage or fixed, limits, expiration, eligible plans	Aviate
Prepaid credits	Credits bought upfront	Aviate Wallet with paid and free credits. Prepaid credits	Aviate

Example prices are demo values.

Worked Example

An Upgrade in the Middle of the Month

A customer on Starter at $49 per month moves to Growth at $199 per month, 4 days into the period.

Illustration, not a screenshot

Upgrade on 6 OctoberPeriod: 2 October to 2 November, 31 days

InvoiceUSD, demo prices

Item	Amount
Growth monthly, 6 Oct to 2 Nov $199.00 × 27 / 31 days	$173.32
Adjustment (subscription change) unused Starter: $49.00 × 27 / 31 days	−$42.68
Invoice total	$130.64

The same figures appear in the Kaui invoice below. From 2 November, the customer is billed $199.00 per month.

Step	What Kill Bill does
1. Preview	Returns the invoice the change would create, with an invoice dry run, so the customer sees $130.64 before confirming
2. Change	Moves the subscription to Growth on 6 October, with an immediate change policy
3. Credit	Credits the 27 unused days of Starter: $42.68
4. Charge	Charges Growth for the same 27 days: $173.32
5. Invoice	Puts both on one invoice of $130.64, collected through the payment plugin

With an end-of-term policy instead, the customer stays on Starter until 2 November and moves to Growth then, with no proration. Catalog rules set the default policy for each change, and an API call can override it.

In the Product

The Catalog, the Upgrade Invoice and the Reports

The Starter plan with its 14-day trial in Aviate, the upgrade invoice from the example in Kaui, and a new subscriptions report from the analytics plugin.

Aviate catalog: the Cloud Platform base product with the plans Starter monthly, Growth monthly and Scale annual. Starter monthly has two phases, a 14-day trial at 0 dollars and an evergreen phase at 49 dollars per month. — The Starter plan in Aviate: a 14-day trial at $0, then $49 per month.

Kaui invoice: Growth monthly from 6 October to 2 November 2026 at 173.32 dollars, an adjustment for the subscription change of minus 42.68 dollars, and an invoice total of 130.64 dollars. — The upgrade invoice in Kaui: $173.32 for Growth, minus $42.68 for unused Starter.

Kaui analytics: a Daily New Subs report showing new subscriptions per day over three months. — The Daily New Subs report from the analytics plugin, in Kaui.

Screenshots from a test environment.

Capabilities

Features for SaaS Billing

Kill Bill

Catalog With Phases

Build each plan from phases: trial, discount, fixed term and evergreen. Plans bill monthly, annually or on other periods.

Catalog →

Kill Bill

Upgrades and Proration

Change plans at once, with a credit for the unused part, or at the end of the term. Preview the invoice with a dry run.

Subscription guide →

Kill Bill

Catalog Versions

Upload a new version with an effective date. Choose when existing subscriptions move to the new prices.

Catalog versions →

Kill Bill

Retries and Dunning

Retry failed payments on your schedule, then block changes or pause the service until the balance is paid.

Dunning guide →

Aviate

Aviate Coupons

Percentage or fixed discounts, with a usage limit, an expiration date and eligible plans.

Coupons →

Aviate

Catalog in Aviate

Create products, plans, phases and prices in a UI, without editing XML.

Catalog →

How It Works

Set Up SaaS Billing in Three Steps

Define the Catalog

Create products, plans, phases and prices, with the change and cancellation rules. Upload the catalog, or build it in Aviate.

Connect Sign-Up and Payments

Your app creates accounts and subscriptions through the API. Install the payment plugin for your gateway and set retries and dunning.

Change Prices Over Time

Upload a new catalog version with an effective date. New customers get the new prices, and you choose when existing ones move.

Why Kill Bill

Why Use Kill Bill for SaaS Billing

Kill Bill

Open Source Core

Kill Bill is open source under the Apache 2.0 license. No fee per transaction, and no share of your revenue.

Subscription Data Stays With You

Deploy on your own infrastructure. Accounts, subscriptions, invoices and payments live in your database.

MySQL

MariaDB

PostgreSQL

Kill Bill

Your Choice of Gateway

Collect through payment plugins, and keep the gateway you have or add another one.

Stripe

Braintree

Plugins →

In Production Since 2010

Deployments on the platform have invoiced billions of dollars since 2010.

Proven in Production

Companies That Run on Kill Bill

Kill Bill powers subscription billing for businesses across industries.

CARFAX

Reduced monthly batch processing from 12 hours to under 1 hour. Integrated Kill Bill with NetSuite and Salesforce for end-to-end quote-to-cash automation.

SaaS Community Platform

Migrated from a SaaS billing provider to Kill Bill for more flexibility to test pricing structures and generate custom analytical reports.

Faceter

Cloud video surveillance provider using Kill Bill for subscription billing at scale with flexible plan configuration.

See all customer stories →

FAQs

SaaS Billing FAQ

Can Kill Bill handle free trials, tiered plans and add-ons?

Yes. Each plan in the Kill Bill catalog is made of phases, such as a free trial, a discount phase and an evergreen phase. Plans can bill monthly, annually or on other periods. Add-on products attach to a base plan, and the catalog says which add-ons each base plan allows.

How does proration work when a customer upgrades in the middle of a period?

With an immediate change, Kill Bill credits the unused part of the old plan and charges the new plan for the rest of the period, on the same invoice. With an end-of-term change, the new plan starts at the next period and nothing is prorated. Catalog rules set the default policy for each change, and an API call can override it.

Can I show a customer what an upgrade will cost before it happens?

Yes. An invoice dry run returns the invoice that a plan change would create, without changing the subscription. Your app can show that amount before the customer confirms.

Can I change prices without affecting existing customers?

Yes. Upload a new catalog version with an effective date. New subscriptions use the new prices from that date, and existing subscriptions keep their current prices. To move existing subscriptions as well, set effectiveDateForExistingSubscriptions on the plan, and the new price applies to them from that date.

Can I give one customer a custom price?

Yes. When you create a subscription or change its plan, you can override the prices of its plan phases for that subscription only.

Does Kill Bill support coupons and promo codes?

Kill Bill open source handles promotions as catalog phases, for example a discount phase for the first months. Aviate Coupons adds coupon codes with a percentage or fixed discount, a usage limit, an expiration date and a list of eligible plans, applied when the subscription is created.

What happens when a card payment fails?

Kill Bill retries the payment on a schedule you set per tenant. Dunning rules then move the account through states based on how long an invoice stays unpaid, with actions such as blocking plan changes or pausing the service and billing until the balance is paid.

Can customers pause or cancel a subscription?

Yes. The API pauses and resumes a subscription bundle, meaning the base plan and its add-ons, and cancels subscriptions. A cancellation can take effect at once or at the end of the term. Catalog rules set the default policy, which an API call can override.

Can I migrate from another billing system to Kill Bill?

Yes. The Kill Bill migration guide describes how to move active accounts and subscriptions with cutover dates, so customers are billed once, while past invoices and payments stay available from the old system.

Is there open source subscription billing software for SaaS?

Kill Bill is an open source subscription billing and payments platform under the Apache 2.0 license. It covers catalogs with trials and phases, add-ons, plan changes with proration, usage, invoices, payments through gateway plugins, retries and dunning. Aviate, the enterprise offering of Kill Bill, adds a catalog UI, coupons, prepaid credits, country tax rates and Level 3 support.

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