I'm developing an app that will have to process payments that can potentially even be international. The first payment gateway that comes to mind is PayPal. Is there a simple guide about how to integrate PayPal with Android Studio or Xcode with Firebase as the backend?
Also, how do I integrate Adaptive Payments?
If you think, PayPal doesn't fit the bill, please feel free to suggest other payment gateways and the way in which I should integrate it.
PS: I have seen the SDK for PayPal but I'd like some first hand knowledge from somebody.