The root of yours is that you keep trying to make this about McDonalds, Amazon, Ebay, Costco or some other contraption instead of the App Store, which is what this is actually is about. Not that the argument matters, because the exact moment when you leave Costco has no bearing on the fact that Costco doesn't restrict what payment processor are used in the digital goods that it sells.
But even if it did, Apple's ToS clearly distinguish between the App Store and the licensed application, and between interactions in the App Store and interactions from within the application. You may not want to make the same distinction in order to be right about some imaginary system that you're thinking about, but this is about the actual App Store, not whatever iMcCostco-Amazon marketplace you've dreamed up.
But even if it did, Apple's ToS clearly distinguish between the App Store and the licensed application, and between interactions in the App Store and interactions from within the application. You may not want to make the same distinction in order to be right about some imaginary system that you're thinking about, but this is about the actual App Store, not whatever iMcCostco-Amazon marketplace you've dreamed up.