39 U.S.C. § 3214
Mailing privilege of former President; surviving spouse of former President
A former President and the surviving spouse of a former President may send nonpolitical mail within the United States and its territories and possessions as franked mail. Such mail of a former President and of the surviving spouse of a former President marked “Postage and Fees Paid” in the manner prescribed by the Postal Service shall be accepted by the Postal Service for transmission in the international mails.