who is liable postman, post office, driver of vehicle that hit post office truck

was in process of receiving package, signed papers, reached into USPS truck, car hit truck, sliding door on usps truck hit son in face, knocked down, ambulance took him to hospital, released later that day, head was ok, received facial stitches in 2 places. in a lot of facial pain. Police called back nexy day also when driver of car was changing his plates on the car that hit the post office truck. Question is onlu the driver of the car liable, or is postman for allowing him to get package himself, and post office for being the vehicle that hit him??