[Drew_]

Your experience with other payment processors on an iPhone are indeed not as good simply because they aren't allowed to be as good. Every single one of your experiences with these alternative payment processors was forced through a Safari browser.

[judge2020]

Nothing is stopping the 'Shop' app or Amazon app from being used for any and all third-party (or fourth party, in this case) payments, as long as they're not selling purely digital goods.