Good point! If people are happy to pay 13€ instead of 10€ for the convenience of using the platform owner's payment system, that should always be an option.

the app maker should have the complete freedom what payment options he offers. if i want you to pay me in stones that is my freedom, the same you are free to not use my app and find another app that offers methods that you like.

Great point. Also true: I already have some level of trust with Apple(I bought their device), not you. So Apple should return more relevant results for me on AppStore or even allow me to filter out those apps, that doesn’t support preferred option of payment. Will you agree with such setting?

I don't mind it being a filter, but I think them sorting them like that by default could be an issue. Would you equivalate it to developers paying for higher search ranking? Apple favoring their own services over others definitely could be an anti-trust issue.

Sure. A reasonable compromise would be that the app developer puts Apple's payment system in the app, for you to use, but all transparently tells you that you have to pay the 30% Apple fee yourself.

So, you'd be shown 2 options. Option 1 would be their payment processor, and option 2 would be Apple's. And you could choose to pay an extra 30% to use Apple's.

Problem solved right? Everyone gets what they want, and pays the appropriate fee.

Card details can be stored in the phone and the OS can offer a payments API through which any payment provider can integrate. Similarly to how the File Provider API works.