(a)  Applicability.⁠—This subchapter shall apply with respect to⁠—

(1)  priority mail;

(2)  expedited mail;

(3)  bulk parcel post;

(4)  bulk international mail; and

(5)  mailgrams;

subject to subsection (d) and any changes the Postal Regulatory Commission may make under section 3642.

(b)  Definition.⁠—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.

(c)  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.