WHILE SERVICING A CUSTOMERS 2016 RAM 1500 PICK UP PULLED TRUCK UP TO DOOR,PLACED KNOB SHIFTER IN TO PARK, GOT OUT AND WALKED UP BESIDE TRUCK AS I GOT TO L/F WHEEL TRUCK STARTED TO MOVE FORWARD TURNED AROUND TO GET BACK IN TRUCK SHOWED TO BE IN DRIVE. TRUCK PROCEDED TO ROLL THROUGH DOOR AND CAME TO A STOP. NO WAN WAS INJURED DAMAGE WAS LIMATED TO TRUCK AND DOOR.