§ 141. Orders, notices and service. Every motor carrier operating pursuant to a certificate or permit issued under this chapter shall file with the commissioner a designation in writing of the name and post office address of a person upon whom or which service of notices or orders may be made. Such designation may from time to time be changed by like writing similarly filed. In default of such designation, service of any notice or order may be made by posting in the office of the commissioner. Whenever notice is given by mail as provided herein the date of mailing shall be considered as the time when notice is served.