Paypal created BrainTree, a payments SDK that integrates PayPal, Venmo, credit and debit cards, and popular digital wallets like Apple Pay and Google Pay. BrainTree has great scalability, fraud protection, and leverage the PayPal network which can streamline any business.
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.
Squareup helps you build and grow your business. Square is a online platform that offers you amazing payment services. their services cut across payment assistance, point of sale, capital and payroll all available by simply creating a Square account.