15% for an end to end service? Citation Please! Paypal, Stripe, Braintree and those guys don't even come close to that. If you are getting charged 5% in fees in this day and age then you are doing something wrong... at least here in the USA.
Apple has a cost of no more than 2.5% for credit card fees, and its probably even a few tenths lower than that. That I will guarantee (seeing as how I have seen companies in the $5million/year range get that percentage from an end-to-end provider, and Apple is quite a bit more than that in volume).
What I think is really ridiculous is that they want to charge 30% for subscription type services. One time app purchases... ok, maybe I can see that. But for a $5/month or more subscription being bought through there its absurd and making it more of a pain for the user because the app developer is forced to break with the single-billing system due to costs. So, simplicity actually hinders the experience here.