This article will help with adding a site address to a quote or invoice that has already been created.
Click into the project that the document is assigned too.
When in the click on either the 'Green Cog' or 'Convert Project' button at the top of the page depending on which one shows for you.
In here select the site address that the document is for then click save.
After this the document should update itself and the site address will appear on the document.