There are 11 US Post Offices in Washington County serving a total of 6,134 residents equating to 558 residents per post office. It's estimated that approximately 9,791 packages are handled across the 11 county post offices annually. Washington County (standard abbreviation: WS) is a county located in the U.S. state of Kansas. As of the 2010 census, the county population was 5,799. Its county seat and largest city is Washington."