39 U.S.C. § 2006: Relationship between the Treasury and the Postal Service

At least 15 days before selling any issue of obligations under section 2005 or 2011 of this title, the Postal Service shall advise the Secretary of the Treasury of the amount, proposed date of sale, maturities, terms and conditions, and expected maximum rates of interest of the proposed issue in appropriate detail and shall consult with him or his designee thereon. The Secretary may elect to purchase such obligations under such terms, including rates of interest, as he and the Postal Service may agree, but at a rate of yield no less than the prevailing yield on outstanding marketable Treasury securities of comparable maturity, as determined by the Secretary. If the Secretary does not purchase such obligations, the Postal Service may proceed to issue and sell them to a party or parties other than the Secretary upon notice to the Secretary and upon consultation as to the date of issuance, maximum rates of interest, and other terms and conditions. Subject to the conditions of subsection (a) of this section, the Postal Service may require the Secretary of the Treasury to purchase obligations of the Postal Service under section 2005 in such amounts as will not cause the holding by the Secretary of the Treasury resulting from such required purchases to exceed $2,000,000,000 at any one time. This subsection shall not be construed as limiting the authority of the Secretary to purchase obligations of the Postal Service under section 2005 in excess of such amount. the Postal Service requests the Secretary of the Treasury to pledge the full faith and credit of the Government of the United States for the payment of principal and interest thereon; and the Secretary, in his discretion, determines that it would be in the public interest to do so.