> Caution:
> Annual subscriptions don't support usage billing.

To tailor your app subscriptions to support more complex pricing models, use usage-based billing. For example, if your app has a default subscription and you want to enable merchants to purchase an optional, add-on subscription for a different product line, you can do the following:

- Modify each subscription to reflect the additional modules that are available.

- Track module billing in your application.

- Use usage charges to bill the merchant for the desired amount at the desired time.

This guide shows how flexible usage-based billing is, and how app developers can leverage it to bill merchants for more complex pricing models. This is especially beneficial for larger app subscribers. The guide uses add-on modules as an example. You can also refer to [an example mutation](/docs/api/admin-graphql/latest/mutations/appSubscriptionCreate#examples-Create_a_subscription_with_add_on_modules).

## Requirements

- Your app can make [authenticated requests](/docs/api/admin-graphql#authentication) to the GraphQL Admin API.

- Your app has the `applications_billing` [access scope](/docs/api/usage/access-scopes).

     Learn how to [configure your access scopes using Shopify CLI](/docs/apps/build/cli-for-apps/app-configuration).

## Step 1: Account for additional spend in a subscription

Create a subscription [for a usage-based pricing plan](/docs/apps/launch/billing/subscription-billing/create-usage-based-subscriptions). You should set the capped amount to cover all the costs that you expect to charge merchants in the 30-day billing interval. For example, to charge the merchant $60 every 30 days for a base subscription, $0.10 per email as a usage-based charge, and an additional $30/day for an add-on module, your usage cap should be $30 and a reasonable average for email usage costs over a 30-day period.

## Step 2: Keep track of billing intervals and usage

In your app, keep track of the billing cycles and/or usage that's associated with any additional charges. For example, if you want to bill biweekly, then you can create a recurring job that [creates a usage charge](/docs/apps/launch/billing/subscription-billing/create-usage-based-subscriptions) for the desired amount every two weeks.