After several hours of troubleshooting, I've found this issue!

The latest SQL Injection patch (http://www.zen-cart.com/forum/showthread.php?t=108428) prevents this contribution from passing any data to the cart.

The contribution code probably needs to be re-written or modified to allow for the changes this patch makes.

So, we now know that anyone using this contribution on Zen Cart 1.3.8a is not patched!

I'll contact one of the Zen Cart team to find out if this is an issue with the contribution or the patch.