The difficulty you'll face for mobile is users man-in-the-middling their own device to find your developer's API key, and then making their own POST requests to increment their credit balance. This is why platforms like the App Store offer the ability to validate receipts of transactions [1].

Probably something you'll need to grow support for if you want this to be a drop-in solution for mobile devs.

[1] https://medium.com/@ronaldmannak/how-to-validate-ios-and-mac...