Questions about Native App GUI integration

Hi Payment-enthusiasts,

A difference with the Web Payment API compared to most other W3C APIs, is that it involves GUI.
However, there's AFACT rather little written about the GUI and its implications for implementers.

Anyway, I have a native app that does the following:
https://cyberphone.github.io/doc/saturn/ui-demo/personalpaymentterminal.html

What would (GUI-wise) have to change in order to use it with the Web Payment API?

thanx,
Anders