39 U.S.C. § 3404

Unsealed letters sent by blind or physically handicapped persons

Unsealed letters sent by a blind person or a person having a physical impairment, as described in section 3403(a)(1) of this title , in raised characters or sightsaving type, or in the form of sound recordings, may be mailed free of postage.

( Pub. L. 91–375 , Aug. 12, 1970 , 84 Stat. 758 .)