November 22, 2012 (CHICAGO) --The U.S. Postal Service is trying out a new premium shipping option for those shopping online. It is teaming up with major retailers to offer same-day package delivery. Right now, the service is being tested only in San Francisco. This follows similar efforts by eBay, Amazon.com and most recently Walmart. The post office says if the trial run is successful, it will expand next year to Chicago and other big cities.