While coding a paypal shopping cart for WordPress, especially one that utilises Instant Payment Notification (IPN), be wary if using plugins that restrict access to WordPress based on IP address. Bad Behaviour is one such plugin, and if you don’t add the necessary IP address to its whitelist things just will not work.
So the Go Live Checklist at paypal is a useful page to know about. Or, as in my case, to find out about!