by delivering a copy to him personally;

by leaving a copy at his dwelling house or usual place of abode with a person of discretion not less than sixteen years of age then residing therein;

by leaving a copy at his principal office or place of business with some person then in charge thereof;

by delivering a copy to an agent authorized by appointment to receive service of such notice;

by mailing a copy by registered or certified mail addressed to contestee at his residence or principal office or place of business. Service by mail is complete upon mailing; or

the verified return by the person so serving such notice, setting forth the time and manner of such service shall be proof of same, and the return post office receipt shall be proof of the service of said notice mailed by registered or certified mail as aforesaid. Proof of service shall be made to the Clerk promptly and in any event within the time during which the contestee must answer the notice of contest. Failure to make proof of service does not affect the validity of the service.