2013-08-30 9:51 am EST
You can add VAT information to your Invoices, but in my research it does not appear to be an included option in OpenCart.
Option 1: Edit your logo
As your logo appears in your invoice, by far the easiest option would be to edit your logo and add your VAT number somewhere near the bottom of the image.
Option 2: Use an extension
I don't have any direct experience with these modules, but it looks like you can use an OpenCart Extension to add your VAT number:
Company & VAT No. in Footer & Invoices - $15
EASILY put your company's trading name, registered company number and VAT registration number within the footer of your website and at the bottom of all your invoices!
detailed vat / custom invoice - $10
When printing invoice adds detailed VAT to every product bought. Customize the invoice adding shop logo and faded logo on invoice background. Ready to be printed from your admin section.
Option 3: Edit your invoice template
In a discussion on this page, Display TAX/VAT number on my invoices, a community member suggested this file - \admin\view\template\sale\order_invoice.tpl. That seems to do the trick because the user asking the question stated, "Many thanks! That is just what I was after!".
I hope this helps!