in the file
process_transaction.php
line 603
<input type="hidden" name="item_number_<?php echo $WA_Store_Cart_Index+1; ?>" id="item_number_<?php echo $WA_Store_Cart_Index+1; ?>" value="<?php echo $WA_Store_Cart->DisplayInfo("ID") ?>" />
is this what needs changing... "ID" to "SKU"
thanks