Today a customer asked me how it comes that there is [an iOS app for purchasing from Amazon][1] while this seems to violate [Apple's App Store review purchasing guidelines][2] (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?

  [1]: https://itunes.apple.com/us/app/amazon-mobile/id297606951
  [2]: https://developer.apple.com/appstore/resources/approval/guidelines.html#purchasing-currencies