39 U.S.C. § 601: Letters carried out of the mail

it is enclosed in an envelope; the amount of postage which would have been charged on the letter if it had been sent by mail is paid by stamps, or postage meter stamps, on the envelope; the envelope is properly addressed; the envelope is so sealed that the letter cannot be taken from it without defacing the envelope; any stamps on the envelope are canceled in ink by the sender; and the date of the letter, of its transmission or receipt by the carrier is endorsed on the envelope in ink. the amount paid for the private carriage of the letter is at least the amount equal to 6 times the rate then currently charged for the 1st ounce of a single-piece first class letter; the letter weighs at least 12½ ounces; or such carriage is within the scope of services described by regulations of the United States Postal Service (including, in particular, sections 310.1 and 320.2–320.8 of title 39 of the Code of Federal Regulations, as in effect on July 1, 2005 ) that purport to permit private carriage by suspension of the operation of this section (as then in effect). Any regulations necessary to carry out this section shall be promulgated by the Postal Regulatory Commission.