Suggestion on admin/includes/classes/editOrders.php

instead of
Code:
$ot_tax_class = constant ($ot_tax_class_name);
use

Code:
        $ot_tax_class = null;
        if (defined($ot_tax_class_name)) {
           $ot_tax_class = constant ($ot_tax_class_name);
        }
to prevent a PHP Warning in case the tax class doesn't exist.