Apple's developer terms require their In-App Purchase API be used for digital "content, functionality, or services" . My app charges user for posting text and picture, is that considered as digital service? can it use external payment mechanisms like stripe instead?
Apple is the one to decide that, not Stripe. Physical goods are not allowed to use IAP. But “business service” is a tricky one. Can this “business service” take place inside the app itself? If so, then it must use IAP. If not, then it is external. But not to worry, if you get it wrong, Apple will correct it for you.
Yes. Anything that is used inside the app itself must use IAP. Look at the bright side, it is practically impossible for you to do this on your own and do it legally.
According to https://stripe.com/docs/mobile/ios, Apple allow ios app to use Stripe to process transactions on physical goods or business service, I am wondering if my app belongs to that "business service" category.
In-App Purchase API vs external payment
