In this case, you need to apply the design change as described here:
http://kb.x-cart.com/display/XDD/How+to+insert+your+HTML+code+into+any+part+of+the+ page
You should use template approach and the view list you need to put your template into is "invoice.head" .
Please, let me know if it works out for you.
Side note: you can test whether it works or not on the thank you page after order placement. It will allow you not to send many emails.
Tony.