I believe papal requires the item ID to be numeric, but you can concatenate the SKU to the name.
edit this line of the process_transaction.php file:
<input type="hidden" name="item_name_<?php echo $WA_Store_Cart_Index+1; ?>" id="item_name_<?php echo $WA_Store_Cart_Index+1; ?>" value="<?php echo $WA_Store_Cart->DisplayInfo("Name") ?>" />
to:
<input type="hidden" name="item_name_<?php echo $WA_Store_Cart_Index+1; ?>" id="item_name_<?php echo $WA_Store_Cart_Index+1; ?>" value="<?php echo $WA_Store_Cart->DisplayInfo("Name") ?> SKU: <?php echo $WA_Store_Cart->DisplayInfo("SKU") ?>" />