How to Get Started with The PayPal API
PayPal is the most popular platform for receiving online payments today. The ease of opening a PayPal account and receiving payments compared to opening a merchant account with a traditional payment gateway is probably the number one reason for its popularity, with a close second being the comprehensive API that PayPal provides for its payment services.
SmashingMagazine has published a really useful article: Getting Started With The PayPal API for building E-Commerce sites or Web Applications. In the article, they will break down some of the techniques and approaches to working with the PayPal API, in order to make integration and troubleshooting simpler and easier.