39 CFR § 310.2 - Unlawful carriage of letters.
(a) It is generally unlawful under the Private Express Statutes for any person other than the Postal Service in any manner to send or carry a letter on a post route or in any manner to cause or assist such activity. Violation may result in injunction, fine or imprisonment or both and payment of postage lost as a result of the illegal activity (see § 310.5).
(i) The letter is enclosed in an envelope or other suitable cover;
(ii) The amount of postage which would have been charged on the letter if it had been sent through the Postal Service is paid by stamps, or postage meter stamps, on the cover or by other methods approved by the Postal Service;
(iv) The cover is so sealed that the letter cannot be taken from it without defacing the cover;
(v) Any stamps on the cover are canceled in ink by the sender; and
(vi) The date of the letter, or of its transmission or receipt by the carrier, is endorsed on the cover in ink by the sender or carrier, as appropriate; or
(i) The activity is in accordance with the terms of a written agreement between the shipper or the carrier of the letter and the Postal Service. Such an agreement may include some or all of the provisions of paragraph (b)(1) of this section, or it may change them, but it must:
(B) Remain in effect for a specified period (subject to renewals); and
(C) Provide for periodic review, audit, and inspection.
(ii) Possible alternative arrangements may include but are not limited to:
(1) Relate to some part of the cargo of, or to some article carried at the same time by, the conveyance carrying it (see § 310.3(a));
(2) Are sent by or addressed to the carrier (see § 310.3(b));
(3) Are conveyed or transmitted without compensation (see § 310.3(c));
(4) Are conveyed or transmitted by special messenger employed for the particular occasion only, provided that not more than twenty-five such letters are conveyed or transmitted by such special messenger (see § 310.3(d)); or
(5) Are carried prior or subsequent to mailing (see § 310.3(e)).