You have to go in and edit skin1/mail/order_date.tpl.
Adding HTML to make it look good is a nice feature; however, you have to remember that this template is also sent out in several of the emails that your customers receive. The HTML in those emails won't display properly unless you click the "HTML Mail" check box in your General Settings. HOWEVER, clicking this check box will also enable HTML mail for all your other emails Б─⌠ so they won't display properly.
My suggestion would be that if you want to just simply print out nice invoices, modify your order.php page so that when mode=invoice it displays a different template (that you design, call it anything you want i.e. invoice_template.tpl) contaning the order_date.tpl variables. This way, you won't have to enable HTML mail which will screw up all your other emails.
If, however, you want to go all out and change all your emails and make them HTML compatible, I have 2 caveats for you:
1) In version 3.3.X there are something like 50+ different emails that can be generated by x-cart
2) Not everyone enables HTML to display in their email programs. So formatting it - especially for AOL users can be tough.
Hope that was clear - good luck everyone
