Today a customer asked me how it comes that there is an iOS app for purchasing from Amazon while this seems to violate Apple's App Store review purchasing guidelines (Apple Developer login needed).

I could give him no answer to his question – it seems that there is a small number of allowed ways to do purchases in apps that Apple permits, even if they do not get a revenue from them.

What kind of purchases does Apple allow in an app that do not go through their purchasing system but through a custom one?