It appears that testing PayPal’s Web Payments Pro in their Sandbox doesn’t work like it would in a live environment. It won’t return error codes for address verification failures out of the box unless you set it up to do so. I was lucky enough to get a response in the developer forum that showed me this blog post on how to test AVS failures.

Also, it doesn’t look like you can test Discover Card or American Express credit cards in the Sandbox either. If there are any PayPal developers out there that come across this blog post, that’s a feature I’d like to see! I need to know that my code works 100% before it goes live!