Title 18. Section 1725. Postage unpaid on deposited mail matter
Whoever knowingly and willfully deposits any mailable matter such
as statements of accounts, circulars, sale bills, or other like
matter, on which no postage has been paid, in any letter box
established, approved, or accepted by the Postal Service for the
receipt or delivery of mail matter on any mail route with intent to
avoid payment of lawful postage thereon, shall for each such
offense be fined under this title.