Starting This Monday change your hourly shop rate to $139.95 per hour and fix their problem the first visit. When cheaper shop prices are mentioned, invite the customer to have the other shop do the work and then bring the car back to your shop for a recheck together. You will point out the sloppy oil change work, poor tire balance, missing/loose bolts ,problems missed ,etc so the that the customer can go back and ask for their money back.