There are 14 US Post Offices in Butler County serving a total of 65,606 residents equating to 4,686 residents per post office. It's estimated that approximately 104,714 packages are handled across the 14 county post offices annually. Butler County (county code BU) is a county located in the southern portion of the U.S. state of Kansas. As of the 2010 census, the county population was 65,880. Its county seat and most populous city is El Dorado."