39 U.S.C. § 3403

1. (a)The matter described in subsection (b) of this section (other than matter mailed under section 3404 of this title) may be mailed free of postage, if—
   1. (1)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;
   2. (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;
   3. (3)the matter may be opened by the Postal Service for inspection; and
   4. (4)the matter contains no advertising.
2. (b)The free mailing privilege provided by subsection (a) of this section is extended to—
   1. (1)reading matter and musical scores;
   2. (2)sound reproductions;
   3. (3)paper, records, tapes, and other material for the production of reading matter, musical scores, or sound reproductions;
   4. (4)reproducers or parts thereof, for sound reproductions; and
   5. (5)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.