The Stripe Haskell Library by David Johnson is written in Haskell. It offers developers Stripe's API features such as transactions, subscriptions, plans, coupons, discounts, invoices as well as others.
The Currency Cloud, cross border payment as a service provider, has enabled developers to integrate international payment processing with an API: XBPConnect. Prior to the Currency Cloud, payment/invoice software required users to leave the platform and engage a third party to process an international invoice. The Currency Cloud allows cross border and multi-currency transactions from a single platform. The API allows developers to integrate the functionality into existing apps and platforms.
Dwolla is an extremely innovative Payment API and the site allows you to transfer money with one of the least payment charges. We have covered Dwolla before here. Here is an interview with Michael Schonfeld, Developer Evangelist of Dwolla API.