I'm not sure. I've found this in the App Store Review Guidelines:
>3.1.5 (a) Physical Goods and Services Outside of the App: If your app enables people to purchase goods or services that will be consumed outside of the app, you must use purchase methods other than in-app purchase to collect those payments, such as Apple Pay or traditional credit card entry.
But if this applies to Bandcamp, then it would equally apply to Amazon... So I don't know.
>3.1.5 (a) Physical Goods and Services Outside of the App: If your app enables people to purchase goods or services that will be consumed outside of the app, you must use purchase methods other than in-app purchase to collect those payments, such as Apple Pay or traditional credit card entry.
But if this applies to Bandcamp, then it would equally apply to Amazon... So I don't know.