Car should not have been released until you were paid for work done. You should have held car until you were paid your due. Was a written agreement sign ahead of time? Did you keep a detailed list of time and materials?
And unless someone has actually done the work you did, which was extensive, and in very short time, and of high quality, requiring extensive skill knowledge, a customer is not going to have a clue. Should not be a surprise customer doesn't have a clue, thats a given, and should be assumed.
The admin over on autobody101 he bills monthly for time materials. And when customers don't pay its a stop work. So strategies to manage risk, minimize losses, and manage customers expectations need to be done.