Service of the subpena shall be made upon the witness no later than three days before the day on which his attendance is directed. A subpena may be served by any person who is not a party to the contested election case and is not less than eighteen years of age. Service of a subpena upon a person named therein shall be made by delivering a copy thereof to such person and by tendering to him the fee for one day’s attendance and the mileage allowed by section
389 of this title. Written proof of service shall be made under oath by the person making same and shall be filed with the Clerk.