Fedex 500 Internal Server Error with MarketPress WP plugin

The MarketPress plugin is causing a “Fedex 500 – Internal Server Error” issue. FedEx says it’s not a problem on their end. The Marketpress developers say they can’t see any problems either. There are no errors in the error log.

The error occurs here:

https://www.jazzandriffshardwarecollection.com/store/shopping-cart/shipping/

If you go directly to that link, you won’t see the error. You have to add a product to the shopping cart, then proceed to checkout. When you get to the page where you enter your shipping information, that’s where you’ll see the error, toward the bottom.

I’ve tried the following:

Renaming the MarketPress plugin directory
Playing around with file permissions
Working with FedEx and the plugin developer to try to fix the error
Checked the memory limit, which is at 256
Any help would be much appreciated.

Thanks.