There are 2 US Post Offices in Union County serving a total of 16,997 residents equating to 8,499 residents per post office. It's estimated that approximately 27,129 packages are handled across the 2 county post offices annually. Union County is a county located in the state of Florida, the smallest in the state. As of the 2010 census, the population was 15,535. The county seat is Lake Butler."