Fees and mileage need not be tendered to a witness upon service of a subpoena issued on behalf of the United States or an officer or agency of the United States, upon service of a subpoena issued on behalf of a defendant represented by a Federal public defender, assistant Federal public defender, or other attorney appointed pursuant to
section 3006A of title 18, or upon service of a subpoena issued on behalf of a party authorized to proceed in forma pauperis, if the payment of such fees and mileage is to be made by the Attorney General under this section.