Post moved....please don't post questions in the Custom Mods forum, that is for posting completed Custom code only. Also, be sure and read the forum rules found here:
http://forum.x-cart.com/viewtopic.php?t=6489
and put your X-Cart version in your signature. Edit profile, enter version, enter password, save.
As far as your question goes, you will want to edit mail/html/order_invoice.tpl.