There are 7 US Post Offices in Yuma County serving a total of 193,622 residents equating to 27,660 residents per post office. It's estimated that approximately 309,042 packages are handled across the 7 county post offices annually. Yuma County is a county located in the southwestern corner of the U.S. state of Arizona. As of the 2010 census, its population was 195,751. The county seat is Yuma. Yuma County comprises the Yuma, AZ Metropolitan Statistical Area. The county borders the Colorado River to the southwest, with three other states across the river: California and the Mexican states of Baja California and Sonora."