Don't you just love customers, might be worth bolting a plank to your boot before kicking them in the ass lol.

Seriously though it might be worth having some form of signage giving diagnostic prices before entering workshop so there is no confusion.
I know a business that had a sign up with prices list as follows: Answers £10, Answers requiring thought £15, Correct Answers £20, when a customer come in asking for information they were pointed to the sign, if they turned away then no grief was to had and no time wasted.

HTH
John