The witness fee shall be $25.00 per diem, and execution shall be issued by the clerk upon affidavit of the witness to enforce payment thereof. The payment of fees shall not be demanded as a condition precedent to attendance; but, when a witness resides outside the county where the testimony is to be given, service of the subpoena, to be valid, must be accompanied by tender of the fee for one day's attendance plus mileage of 20¢ per mile for traveling expenses for going from and returning to his or her place of residence by the nearest practical route. Tender of fees and mileage may be made by United States currency, postal money order, cashier's check, certified check, or the check of an attorney or law firm. When the subpoena is issued on behalf of the state, or an officer, agency, or political subdivision thereof, or a defendant in a criminal case, fees and mileage need not be tendered.