Here goes the workaround:
xpc_func.php.patch.txt
The patch is to be applied in X-Cart. See how to apply patches in X-Cart:
http://help.x-cart.com/index.php?title=X-Cart:Applying_Patches
http://help.x-cart.com/index.php?title=X-Cart:To_apply_a_patch_manually
Please notice that this is not a complete solution. It is a patch that removes sending PayPal line items from X-Cart to X-Payments and thus removes this issue.
The complete solution will provided with release of X-Cart 4.5.5. Reason: this is a complex issue that needs fixing in different places of X-Cart. Our techs will provide the patches for the issue a bit later as it needs some time to be investigated and resolved.
Dinny, also I ask you to update your signature to show what version of X-Cart and what X-Payments plan you are using. This information will help forum members help you faster and better.