that's a good idea on their part. That way you couldn't get hit with a chargeback on labor. If someone decides they don't like the labor cost/quality they can dispute the charge with their card holder. Then until the dispute is cleared up, you are out the money.
Thread highjack: In a totally different, non automotive business,
I had a customer dispute a charge AFTER i had refunded their cc account. It can 2-3 days for a credit to show up on someones account. They thought it was instant and got PO'd and disputed the charge. Until it got cleared up, I was out 2x the $.

. The stupid part is that this person had signed a membership agreement with a 30 days notice cancellation policy. 3 months go by without seeing them as their monthy charge occurs and their in-house credit builds. Then they call all PO'd and demand refund, claim they never signed a contract, blah blah blah. So to be nice, we agree to refund them, even though we are not obligated. That's when they disputed the charges. At some point during this mess they call us back and say they will just come in and start their membership up again in about 1.5 months when they are back in town. We then told them, "sorry, but your money is no good here."

moral of the story: Contrary to popular belief, the customer is not always right, and some need to be told that fact. No one has the right to mistreat me or my staff and continue to use our service.