The commission has power to hold hearings, subpoena witnesses, compel their attendance, administer oaths, take the testimony of any person under oath, and in connection therewith, to require the production for examination of any books or papers relating to any matter under investigation or in question before the commission. The commission may make rules as to the issuance of subpoenas by individual members, as to service of complaints, decisions, orders, recommendations and other process or papers of the commission, its member, agent, or agency, either personally or by registered mail, return receipt requested, or by leaving a copy thereof at the principal office or place of business of the person required to be served. The return post office receipt, when service is by registered mail, shall be proof of service of the same.