How to write a demand Letter with the following information?

On December 1, 2017, Mike was driving his vehicle on the 401 westbound when he got into a minor collision. His front right bumper and front right tire were damaged. He had his vehicle towed to a local garage for repairs. The repairer gave Mike an oral estimate of $1,400 to fix his vehicle. Mike agreed and the repairer took a Post-It note and wrote the estimate on it. Mikethanked the repairer and stated that he didn't want the repairer to get any work donebeyond the estimated amount. The repairer agreed. The next day when Mike came to pick up his car, the repairer told him that they had to get additional work done and that they would have to charge him an extra $400 for the extra repairs carried out. Mike refused to pay the extra fee. As a result, the repairer sued Mike and put a lien on the car.