Billing is a hard issue since there is no common API to do that. All payment gateways are different. And what is most important, it lot of cases it requires some application logic, on how exactly you want to bill the user: for example partly static, partly dynamic.

What API gateway can do, is to provide you with a way to set a quota for a given user, track analytics, and have it all accessed in a programmatic way, so with a few API queries, you can know how much to bill the user. At least that's how most of the Gateways, like Tyk, do.