The question is, do you have your vehicle or is it still at the shop? If they still have it I would get an attorney to write a demand letter for the car and regarding the bill. If not, you can simply refuse to pay and if they report it to the credit bureaus then sue them, or if they sue you, file a counterclaim. Or, you could have an attorney try to head any of that off by sending a letter threatening to file suit for this fraud they are trying to commit if they do not acknowledge the debt is paid in full. Finally, you could sue them for a crapping repair and paint job and deal with it all in that suit.
