There are 15 US Post Offices in Pinal County serving a total of 219,764 residents equating to 14,651 residents per post office. It's estimated that approximately 350,768 packages are handled across the 15 county post offices annually. Pinal County is a county located in the central part of the U.S. state of Arizona. As of the 2010 census, the population was 375,770, making it the third-most populous county in Arizona. The county seat is Florence. The county was founded in 1875. Pinal County contains parts of the Tohono O'odham Nation, the Gila River Indian Community and the San Carlos Apache Indian Reservation, as well as the entirety of the Ak-Chin Indian Community. Pinal County is included in the Phoenix-Mesa-Scottsdale, AZ Metropolitan Statistical Area. Suburban growth southward from greater Phoenix has begun to spread into the northern parts of the county; similarly, growth northward from Tucson is spreading into the southern portions of the county. The Pinal County cities of Maricopa and Casa Grande, as well as many unincorporated areas, have shown accelerated growth patterns in recent years; such suburban development is likely to continue for the foreseeable future."