There are 23 US Post Offices in Collin County serving a total of 531,463 residents equating to 23,107 residents per post office. It's estimated that approximately 848,274 packages are handled across the 23 county post offices annually. Collin County is a county located in the U.S. state of Texas. As of the 2010 United States Census, the county's population was 782,341, making it the seventh-most populous county in Texas and the 63rd largest county by population in the United States. Its county seat is McKinney. Collin County is part of the Dallas-Fort Worth-Arlington, TX Metropolitan Statistical Area. A small portion of the city of Dallas is located in the county."