[nikolayasdf123]

wouldn't you want to create payment gateway and abstract away logic such that client is agnostic of payment processes and backend confirms internal payment process into external specific ones? (in worst case redirect to other apps with universal links or webview)

[bastawhiz]

You're not doing it in a browser window. You're integrating against device APIs like NFC, you've got custom UI (which you probably want to be native), you've got stuff like camera access to read QR codes and OCR a credit card. Want to pay by topping up a wallet at 7-Eleven? Now you need a map to show where the nearest one is.