Hi Jeff,
We researched the issue (thank you for sharing so many details, this did help us) and found a bug in our software. It has been already fixed and the fix is now waiting for internal testing. We are finalizing XP v3.1 now where the fix is going to be included, but if you need a patch before we release XP v3.1.0 we can share it with you via your Helpdesk account at
https://secure.x-cart.com (just post a ticket about that with "Attn for Alex Mulin" as subject).