Hey @alexw1, our App Billing API does have some features that would allow you to set up usage charges that can be customized. Essentially, you would set up a recurring subscription and create pricing plans to match the payment "tiers" per usage type that you'd like to offer.
In terms of actually tracking the transaction volume, you would have to build tracking methodology on your end, however this could be set up with a combination of webhook subscriptions and redundancy polling that you could monitor per shop (more on how Shopify webhook subscriptions work here).
There is a bit more information in the documentation here on how to set up those recurring usage charges too as a starting point. Hope this helps - let us know if we can clarify anything further on our end!