Stripe - Online payment processing for internet businesses. Stripe is a suite of payment APIs that powers commerce for online businesses of all sizes. Use Stripe’s payment platform to accept and process payments online for easy-to-use commerce solutions.
Wirize - Payment checkout for freelancers
ACH payments with Stripe - Accept bank payments with a few lines of code.
Giftbit Payment API - Stripe for gift cards
Recurring Payments - No need to create invoices. No need to remind clients to pay
Mastering Modern Payments - 2nd edition, fully revised, new content, new web reader.