It shall be the duty of every carrier subject to this chapter to designate in writing an agent in the District of Columbia, upon whom service of all notices and process and all orders, decisions, and requirements of the Commission may be made for and on behalf of said carrier in any proceeding or suit pending before the Commission, and to file such designation in the office of the secretary of the Commission, which designation may from time to time be changed by like writing similarly filed; and thereupon service of all notices and process and orders, decisions, and requirements of the Commission may be made upon such carrier by leaving a copy thereof with such designated agent at his office or usual place of residence in the District of Columbia, with like effect as if made personally upon such carrier, and in default of such designation of such agent, service of any notice or other process in any proceeding before said Commission, or of any order, decision, or requirement of the Commission, may be made by posting such notice, process, order, requirement, or decision in the office of the secretary of the Commission.