I thought I would post this in case it benefited others.
Problem: I tried several times to create a PayPal button in the sandbox. Each time, the generated code did not contain any reference to the sandbox; it always generated code for a "live" site. This was even the case when I walked through the process on the phone with PayPal tech support. Naturally, then, there way no way to test the PayPal payment functionality and I got all sorts of errors.
Root Cause: Not entirely sure, but it's something on the PayPal side when you go to create the button in the sandbox.
Solution: I w