39 U.S.C. § 3403

the matter is for the use of the blind or other persons who cannot use or read conventionally printed material because of a physical impairment and who are certified by competent authority as unable to read normal reading material in accordance with the provisions of sections 135a and 135b of title 2;

no charge, or rental, subscription, or other fee, is required for such matter or a charge, or rental, subscription, or other fee is required for such matter not in excess of the cost thereof;

the matter may be opened by the Postal Service for inspection; and

the matter contains no advertising.

reading matter and musical scores;

sound reproductions;

paper, records, tapes, and other material for the production of reading matter, musical scores, or sound reproductions;

reproducers or parts thereof, for sound reproductions; and

braille writers, typewriters, educational or other materials or devices, or parts thereof, used for writing by, or specifically designed or adapted for use of, a blind person or a person having a physical impairment as described in subsection (a)(1) of this section.