The following standards apply to single-piece First-Class Mail:
a. Each piece must have a delivery address but is not required to bear the 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. There are no sorting requirements for single-piece 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 bundled. Bundling of letter-size pieces is not required if they fill a letter tray. Metered mail and permit imprint mail may not be bundled or trayed together.