The Authorize.net iOS SDK by Authorize.net offers API interaction with the aim to interact with the gateway. Includes installation and guides to implement with ApplePay. Developer Brian McManus mentions that the kit can be useful to build Mobile Point-of Sales applications.
Square now has an E-commerce API that brings Square services to third party websites. Square is usually associated with its mobile credit card reader and complementary software. With the E-commerce API, merchants can use their own systems with all the benefit of Square's modern payment services.
Celery, flexible ecommerce solution provider, allows users to accept pre-orders without charging the customer until the product is ready to ship. On the flip side, customers also have the capability to pre-pay for goods and services. In addition to flexible payment options, Celery allows developers to integrate its functionality with third party apps and workflows via the Celery API.
Payment options are not limited to cash, checks, or credit cards anymore. ECommerce and mobile tech have transformed the ways people buy goods and services in the past few years. Driving the new options are APIs. Here's a list of top APIs for Payments, according to traffic on ProgrammableWeb.