I may have lost track of context here. But at least in my contract, it says due "upon" completion because it denotes payment is due right then and there whenever possible, not just any day "after". It's partly to clarify why the $50 late fee kicks into gear 15 days following if someone happens to delay that long.once you are finished you are "complete". so in effect he writes after completion unless I am missing something.
In 30 years, I think only 3 people have paid a late fee. They were all regular customers who lost track of paper work and ended up paying like 30 days after the work was done. When 15 days nears, I've always given a call as a heads-up too, so the late fee seems fair for the small extra hassle.