39 U.S.C. § 3631

1. (a)This subchapter shall apply with respect to—subject to subsection (d) and any changes the Postal Regulatory Commission may make under section 3642.
   1. (1)priority mail;
   2. (2)expedited mail;
   3. (3)bulk parcel post;
   4. (4)bulk international mail; and
   5. (5)mailgrams;
2. (b)For purposes of this subchapter, the term “costs attributable”, as used with respect to a product, means the direct and indirect postal costs attributable to such product through reliably identified causal relationships.
3. (c)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.