39 U.S.C. § 3621: Applicability; definitions

first-class mail letters and sealed parcels; first-class mail cards; periodicals; standard mail; single-piece parcel post; media mail; bound printed matter; library mail; special services; and single-piece international mail, Mail matter referred to in subsection (a) shall, for purposes of this subchapter, be considered to have the meaning given to such mail matter under the mail classification schedule. Except as provided in subparagraph (B), rates of postage may not be established, under subchapter II [now I] of chapter 36 of title 39, United States Code, in a manner designed to allow the United States Postal Service to receive through revenues any portion of the additional revenues (referred to in section 2401(d) of such title, as amended by paragraph (1)(E)) for which amounts are authorized to be appropriated under such section 2401(d). If Congress fails to appropriate an amount authorized under section 2401(d) of title 39 , United States Code (as amended by paragraph (1)(E)), rates for the various classes of mail may be adjusted in accordance with the provisions of subchapter II [now I] of chapter 36 of such title (excluding section 3627 thereof) such that the resulting increase in revenues will equal the amount that Congress so failed to appropriate.”