References
- Adding PayPal Checkout to Your 3rd-party Shopping Cart (written by PayPal)
- PayPal Developer's Sandbox
- PayPal Developer forums - handy place to ask your questions about PayPal integration.
- Information about PayPal's IPN (Instant Payment Notification)
- PayPal Hacks by Shannon Sofield, Dave Nielsen & Dave Burchell. O'Reilly Press (book)