Service by the Commission of all notices and reports under this Article shall be made by personal service or by registered or certified mail at the residence or principal place of business of the party served. A return made and certified by the person making such service, or a returned Post Office receipt in the case of registered or certified mail, shall constitute proof of service. (M.C. 1963, § 27.1210; amd. Ord. 0-68-20)