There are 13 US Post Offices in Solano County serving a total of 380,505 residents equating to 29,270 residents per post office. It's estimated that approximately 607,328 packages are handled across the 13 county post offices annually. Solano County is a county located in the U.S. state of California. As of the 2010 census, the population was 413,344. The county seat is Fairfield. Solano County comprises the Vallejo-Fairfield, CA Metropolitan Statistical Area, which is also included in the San Jose-San Francisco-Oakland, CA Combined Statistical Area. Solano County is the northeastern county in the nine-county San Francisco Bay Area region. A portion of the South Campus at the University of California, Davis is in Solano County."