Apple and Google (and others) also allow loading credit using physical cards that you purchase at retail stores. I believe the stores get around a 5-10% margin on these, i.e. you buy a $100 gift card, and the store only pays Apple/Google $90-95.

There is a definite cost to all these so 0% is unreasonable. Epic is trying to be the "good" games distribution store and they reportedly take a 12% cut. Something around there, maybe down to 10% would be a reasonable place for Apple/Google to be.