I looked over it briefly and saw some things about merchant validation and other events. I haven't done much with payment handling other than use APIs like PayPal. My gut feeling is that the asynchronous nature of this is going to make it difficult for users to implement properly.

Is there a synchronous abstraction for payments? So basically the user would submit a JSON object with the payment information and get a single atomic approve/deny response that is final?

If so, then I'd like to see a wrapper for this that encapsulates all of the async minutia into a sync protocol.

If not, then I think that reveals the underlying complexity of payment handling which is due to its asynchronous nature. There are so many reasons for payments to be rejected, returned or fail outright that perhaps nobody gets it all right. In the end, somebody has to moderate it and make it a manual process at some level.

So that's the part that that I'd like to see someone solve, regardless of the interface. Maybe Stripe or Square already have? Sorry if I'm using the wrong terminology here, I haven't done much merchant stuff, but have a nose for the fundamentals that make these types of things complex.