Thank you for your question, I believe I found a solution for you. On the left side of your AllProWebTools console go to settings->invoicing. Once there click on the Invoice Email Tab Then there is a box where you may enter text or any additional data. In that box type in what you need: "we need payment within 7 days to guarantee merchandise" At the bottom of the page click on the green box that says save. That should now be your new standard invoice email and will go out with all invoices.
Here is a video with some additional information that I think may help you: