18 U.S.C. § 1723: Avoidance of postage by using lower class matter

Matter of the second, third, or fourth class containing any writing or printing in addition to the original matter, other than as authorized by law, shall not be admitted to the mails, nor delivered, except upon payment of postage for matter of the first class, deducting therefrom any amount which may have been prepaid by stamps affixed, unless by direction of a duly authorized officer of the Postal Service such postage shall be remitted. Whoever knowingly conceals or incloses any matter of a higher class in that of a lower class, and deposits the same for conveyance by mail, at a less rate than would be charged for such higher class matter, shall be fined under this title.