The service I am building is usage-based, i.e.

  * I am charging per second the service was used, e.g. USD 0.01/second
  * I would like to give credits upon sign-up.
  * There is no monthly minimum.

At the moment, I am thinking that I can achieve this with just a few tables:

  * `billing_account (id, billing_rate_id, running_balance)`
  * `billing_rate (id, rate)`
  * `account_credit (id, billing_account_id, timestamp, amount, function_execution_id)`
  * `account_debit (id, billing_account_id, timestamp, amount, source [platform_credit or stripe])`

When a user creates an account, I would create an entry in `billing_account` and associate whatever the current `billing_rate`. I would also create an `account_debit` entry with `source=platform_credit` and update `billing_account` `running_balance` value to reflect their balance after the `platform_credit`.

Then whenever they run a function that costs them, I create `account_credit` account with an entry equal to the amount they spent and update `billing_account` `running_balance` value.

When they top-up their account, I would just add entry to `account_debit` and update the balance again.

This appears to cover all my use cases, but I wanted to check with anyone who's designed such database schemas before.