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.
Amazon in app purchasing (IAP) API allows your app to present, process, and fulfill purchases of digital content and subscriptions within your app. With Amazon In-App Purchasing (IAP), your app's users can purchase various types of digital items within your app, such as extra lives for a game or a subscription to premium content.
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.