Am I allowed to use in my iOS app a prepay system, where user can upload bitcoin to use in-app premium services? I want to avoid pay 30% for Apple.
According to the App Store Review Guidelines (under section 11.2), anything that circumvents the in-app purchase API can and likely will be pulled and/or rejected. Going around Apple and using your own system would be a bad idea with the possibility of you being ejected from the developer program.
Hope this helps!