Charges, petitions, orders and other process and papers of the board, its members, agent or agency, may be served personally, by regular mail, or by leaving a copy at the principal office or place of business of the person to be served. The verified return by the server, setting forth the manner of such service, or the return post office receipt when registered and mailed as aforesaid, shall constitute proof of service. Final orders issued by the board shall be served upon the parties by registered or certified mail.