39 U.S.C. § 3204: Restrictions on use of penalty mail

a request therefor has been previously received by the department or establishment; or its mailings is required by law. enclosures reasonably related to the subject matter of official correspondence; informational releases relating to the census of the United States and authorized by title 13; matter concerning the sale of Government securities; forms, blanks, and copies of statutes, rules, regulations, instructions, administrative orders, and interpretations necessary in the administration of the department or establishment; agricultural bulletins; lists of public documents offered for sale by the Superintendent of Documents; announcements of the publication of maps, atlases, and statistical and other reports offered for sale by the Federal Power Commission as authorized by section 825k of title 16 ; or articles or documents to educational institutions or public libraries, or to Federal, State, or other public authorities.