So wait. Let me get this straight, I send an invoice for deposit (invoice #1). I complete the services requested and send the Final Invoice (invoice #2). 
Now, if they want to include a voluntary tip, I am required to ask what that amount is so I can send it in an additional invoice (invoice #3).

How hard is it to add a voluntary tip section to every invoice sent out by the company? It is voluntary to pay it, leaving it to the customers  discretion to pay it… Way better then the awkward, “how much do you want to tip?” conversation…