PayPal Checkout SDK and Braintree Direct SDKs listed. Checkout is an in-app SDK to smooth the checkout process in PayPal. Braintree works with Android and iOS. It is a recommended way to accept PayPal, credit, or other payment methods.
Stripe brings simplicity interms of making payments in your iOSapp. Stripe is opensource and offers you an API that allows for easy synching payments with the powerful, customizable, UI elements available out-of-the-box to help collect your users' payment details.