39 U.S.C. § 3621

(a) Applicability .— This subchapter shall apply with respect to— (1) first-class mail letters and sealed parcels; (2) first-class mail cards; (3) periodicals; (4) standard mail; (5) single-piece parcel post; (6) media mail; (7) bound printed matter; (8) library mail; (9) special services; and (10) single-piece international mail, subject to any changes the Postal Regulatory Commission may make under section 3642. (b) Rule of Construction .— 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.