There are 6 US Post Offices in Clay County serving a total of 14,487 residents equating to 2,415 residents per post office. It's estimated that approximately 23,123 packages are handled across the 6 county post offices annually. Clay County is a county located in the U.S. state of Illinois. As of the 2010 census, the population was 13,815. Its county seat is Louisville. In 1950, the U.S. Census Bureau placed the mean center of U.S. population in Clay County."