There are 3 US Post Offices in Madison County serving a total of 37,552 residents equating to 12,517 residents per post office. It's estimated that approximately 59,937 packages are handled across the 3 county post offices annually. Madison County is a county located in the U.S. state of Idaho. As of the 2010 census, the population was 37,536. The county seat and largest city is Rexburg. Madison County is part of the Rexburg, ID Micropolitan Statistical Area, which is also included in the Idaho Falls-Rexburg-Blackfoot, ID Combined Statistical Area."