M110 describes the mail preparation requirements for single-piece First-Class Mail.
[4-3-03] The following standards apply to single-piece rate First-Class Mail:
a. Each piece must have a delivery address but is not required to bear the rate marking First-Class or First-Class Mail.
b. No minimum quantity is required unless postage is paid with a permit imprint (in which case the mailing must contain at least 200 pieces or 50 pounds of pieces).
c. [4-17-03] There are no sortation standards for single-piece rate First-Class Mail, but five or more letter-size pieces bearing metered postage and all pieces bearing permit imprints must be faced with the addresses in one direction and packaged. Packaging of letter-size pieces is not required if they fill a tray (see M011.1.3b). Metered mail and permit imprint mail may not be packaged or trayed together.
DMM Issue 58 (8-10-03)