18 U.S.C. § 1719 : US Code - Section 1719: Franking privilege

Search 18 U.S.C. § 1719 : US Code - Section 1719: Franking privilege

Whoever makes use of any official envelope, label, or indorsement
authorized by law, to avoid the payment of postage or registry fee
on his private letter, packet, package, or other matter in the
mail, shall be fined under this title.