ShopSmart, the shopping magazine from the publisher of Consumer Reports, conducted a test of shipping options, and found the USPS to be the best value:
Ship it for less. ShopSmart compared shipping a 6-pound medium-sized box from New York to San Francisco using FedEx, the United Parcel Service and the U.S. Postal Service. It checked standard and express delivery times. The cheapest option: the Postal Service’s two-day Priority Mail Service, using its free flat-rate box (for up to 70 pounds). It was $40 less than the competition for the same two-day delivery. The Postal Service was also the cheapest option in other scenarios ShopSmart tried.