There are 9 US Post Offices in Douglas County serving a total of 22,719 residents equating to 2,524 residents per post office. It's estimated that approximately 36,262 packages are handled across the 9 county post offices annually. Douglas County is a county located in the U.S. state of Illinois. As of the 2010 census, the population was 19,980. The county seat is Tuscola.