I look at these boys as independent contracters. If they do what they advertise, i am happy to pay them the amount we discused ahead of time. There have been times when the boy could't keep it up, and i told him i would give him another 10 or 15 minutes to get it up and stay hard.if not i let them know that we will be done and i give them a portion of the money for time spent.
Same thing i would do if i hired a plumber or painter.