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

((1)) priority mail;

((2)) expedited mail;

((3)) bulk parcel post;

((4)) bulk international mail; and

((5)) mailgrams;

((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.

((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.