There are 12 US Post Offices in Sumner County serving a total of 22,259 residents equating to 1,855 residents per post office. It's estimated that approximately 35,528 packages are handled across the 12 county post offices annually. Sumner County (standard abbreviation: SU) is a county located in the U.S. state of Kansas. As of the 2010 census, the population was 24,132. Its county seat is Wellington."