I am building an app that offers scheduling services for retail/restaurants. I want to use the Braintree iOS UI in the app to let the user pay for the subscription to use the app. Is this possible, or will the app get rejected because this use case falls outside of the In-App purchase guidelines?
In reading the App Store Review Guidelines, specifically section 11 (about purchasing), I'm pretty sure it would be rejected under rule 11.2:
11.2 Apps utilizing a system other than the In-App Purchase API (IAP) to purchase content, functionality, or services in an App will be rejected
I think Apple would categorize a subscription as a 'service', and PayPal isn't IAP, so your app would be rejected. Use IAP.