Whenever the officer or individual who has served subpoenas issued by the district attorney for any term of court shall make return thereof as required by law, he shall be required to return all subpoenas, received within a reasonable time prior to such return day, at one and the same time; and in making such return of subpoenas he shall receive mileage only for trips actually and necessarily made in returning such subpoenas and not mileage on each separate subpoena returned. In order to carry into effect the provisions of this section, the district attorney in issuing subpoenas shall, so far as conveniently may be, mail or cause to be delivered subpoenas going to the same officer or individual for a term of court at one and the same time.