There is no minimum for the number of pieces that may be sent at First-Class Mail single-piece prices. If you have at least 500 pieces in the same size category [letter-size or oversize envelopes (flats)], you may be able to send them at lower First-Class Mail presort prices. For further information, see DMM 233 at https://pe.usps.gov/text/dmm300/233.htm. To send USPS Marketing Mail, you need to have at least 200 pieces or 50 pounds in the same size category. For further information, see DMM 243 at https://pe.usps.gov/text/dmm300/243.htm.
