Flats and Large Envelopes

A One Stop Shipping Solution for All Postal Classifications

Whether mailing flats, large envelopes, letters,  small parcels, or anything else, our expert team can provide an efficient mailing solution for you. Through our partnership with the USPS, we will identify which classification will allow you to ship your items at the lowest cost, and leverage our OSM Premium Network to provide residential delivery in a 2-5 day time frame. 

What Classifies as a "Flat?"


The USPS classifies large envelopes, or flat mail, as a mail piece that is too long in at least one direction to be classified as a letter, but small enough that it can be sorted by a sorting machine. More specifically, a piece of mail can be a flat if any of the below are true:

  • its shortest side is longer than 6.5 inches but not longer than 12 inches
  • its longest side is longer than 11.5 inches but not longer than 15 inches
  • its more than 0.25 inch thick but not more than 0.75 inch thick


In addition to meeting certain size specifications, a flat must not be too rigid that it cannot pass through automated sorting machines.  For more information on whether your items will meet flexibility requirements, see the USPS flex test.

Common Items that Ship as Flats

Common examples of items that might ship as flats include:

  • Advertisements
  • Catalogs
  • Information kits
  • Guidebooks
  • Membership materials
  • Questionnaires
  • Welcome packets