Flats Sequencing System (FSS) is a new generation mail-sortation system being deployed by the United States Postal Service to automate the sorting of flat-sized mail into delivery sequence order at high speeds.

FSS is a fully integrated two-pass system consisting of several major components. The key components of the system include: 1) automatic high speed feeders, 2) a dolly induction system, 3) carousel sorter, 4) tray staging, 5) integrated tray converters, and 6) a standalone mail prep sub-system. Combined, these components can sequence flat mail at 16,500 pieces per hour. It is this throughput that makes the system so valuable when compared to how the Postal Service currently sequences flat mail manually.

FSS machines are equipped with a self-contained staging and material handling system; at the end of pass-one sorting an automatic sweep occurs and all trays are returned to the feed area in correct order for immediate pass-two sequencing. At the end of the second pass, mail is automatically swept, placed into street trays, and discharged onto mail transport equipment that will be dispatched to the dock, and ultimately loaded onto trucks destined for the delivery units.