For the purpose of any purchase of the obligations of the Postal Service, the Secretary of the Treasury is authorized to use as a public debt transaction the proceeds from the sale of any securities issued under chapter 31 of title 31, as now or hereafter in force, and the purposes for which securities may be issued under chapter 31 of title 31, as now or hereafter in force, are extended to include any purchases of the obligations of the Postal Service under this chapter. The Secretary of the Treasury may, at any time, sell any of the obligations of the Postal Service acquired by him under this chapter. All redemptions, purchases, and sales by the Secretary of the obligations of the Postal Service shall be treated as public debt transactions of the United States.