There are 7 US Post Offices in Douglas County serving a total of 123,043 residents equating to 17,578 residents per post office. It's estimated that approximately 196,390 packages are handled across the 7 county post offices annually. Douglas County is the seventh-most populous of the 64 counties of the U.S. state of Colorado. As of the 2010 census, the population was 285,465. The county seat is Castle Rock. Douglas County is part of the Denver-Aurora-Lakewood, CO Metropolitan Statistical Area. It is located midway between Colorado's two largest cities, Denver and Colorado Springs. Douglas County has the highest median household income of any Colorado county or statistical equivalent. It is ranked ninth nationally in that category, and has the highest of any county or equivalent not in the eastern US."