If your car suffered damage and you have incurred out of pocket costs to repair it (e.g. you didn't have collision insurance; or had a high deductible; etc.), you can sue the Post Office and the driver for those costs. If you are going to do this, retain an attorney to help you right away: there are special rules and a shorter timeline for suing a government agency or employees than for suing a private business or person, and if you wait too long or don't do things right, you will lose you chance to sue.