In the desktop app, under billing. Reopen the bill, update the information, and then approve the bill. 

If they are paying online, you will have to resend the invoice through the web browser.

Go back to the actual inspection... Then expand the visit count section.

In the visit count, open the created bill. 

Click the "email" button. then send it to the recipient. This will send the invoice with the online payment link in the PDF if this is established.