There are 6 US Post Offices in Union County serving a total of 12,853 residents equating to 2,142 residents per post office. It's estimated that approximately 20,515 packages are handled across the 6 county post offices annually. Union County is a county located in the U.S. state of Iowa. As of the 2010 census, the population was 12,534. The county seat is Creston."