You can generate a tax invoice on postage labels after 5 days (once the option to cancel them is gone).

Go to Selling Manager, and click on 'Manage Postage labels".

In the dropdown menu of any labels that are more than 5 days old, will be the option to print a tax invoice.

This is what they look like, and you can see it already includes GST (the blank boxes are where my name and address appears, and the tracking number).