For in-app purchases, you must use Apple StoreKit. Stripe/Braintree are not options for allowing customers to subscribe to your service natively.
As another person mentioned, you can apply for the Apple Small Business programme, which reduces the commission to Apple to 15%.
You can provide additional payment options to customers via your own website, but this generally results in a poor user experience and isn't worth it for smaller apps (think Spotify).
Using any other payment method risks rejection. Please ensure that Apple StoreKit is used.
Stripe is a good solution if you want to accept payments for goods/services (think GrubHub), but that doesn't sound like what you're looking for.