39 U.S.C. § 3213
Seeds and reports from Department of Agriculture
Seeds and agricultural reports emanating from the Department of Agriculture may be mailed—
(1)
as penalty mail by the Secretary of Agriculture; and
(2)
during the 90-day period immediately following the expiration of their terms of office, as franked mail by Members of Congress.
( Pub. L. 91–375 , Aug. 12, 1970 , 84 Stat. 754 ; Pub. L. 97–69, § 5(b) , Oct. 26, 1981 , 95 Stat. 1043 .)