Has anyone been able to customise their 'Thank You' emails beyond the added text in Symbiostock > Settings ?

I've done some customising to the pre-header area in purchase_thank_you.html but it doesn't appear on the sent email. I want to put enough info on it to make it a legal GST Tax Invoice.

ETA:
Never mind, it's the welcome_template.html that needs to be modified! Next question, how do I make the extra 'Tax Invoice' header appear only on the 'Thank You' email?

If you are unable to do it within Symbiostock could you create a standard Invoice/Receipt email and send that out after the licensing is complete, would mean you can edit it to suit the customer as well - unless you start to get too many sales a day to make it worth while (which would be great)