There are 6 US Post Offices in Costilla County serving a total of 4,881 residents equating to 814 residents per post office. It's estimated that approximately 7,791 packages are handled across the 6 county post offices annually. Costilla County is the ninth-least populous of the 64 counties in the U.S. state of Colorado. As of the 2010 census, the population was 3,524. The county seat is San Luis, the oldest town in Colorado."