Legal ways to collect your bill for auto repairs?

Not sure what your asking? Are you saying the guy is letting the car go without collecting payment? If so he'll learn not to do that again in short order.
Or are you saying he is repairing them and the customer refuses to pay him? Simple, do not let them have the vehicle , in Fl. after 30 days your allowed to put a mechanics lien and collect storage of up to like 3.00 a day? (Not sure it may be more now)