It does seem that a majority of people use usps
Yes, because of cost. I used to work in a fitness warehouse, and almost every product that was ordered left via Fed-Ex and weighed over 65 lbs. Also, I can attest that the average shipping cost was in the $45 range, and shipping 50 - 100 items per day can quickly add up. Customer Service pieces, like washers, cables, shirts, etc., were at one time shipped Fed-Ex also, but the costs got eaten by the company, and just by switching the C.S. to U.S.P.S. or Fed-Ex Ground we could cut the costs up to half. That is another reason why Fed Ex is currently struggling and why I chose to end my employment there. The growth is international and Ground, and a study was done and proven that in most major markets, the time it took an item to leave the shipper's dock to the time it took to reach the customer's door could be achieved by sending the item Fed Ex Ground vs. Fed Ex Express could be achieved almost 60% of the time and at 1/4 less the cost. To put another way, Fed Ex Ground and Fed Ex Express (air division) compete against each other, and while more and more shippers are sending packages via Ground, the time it actually takes to get the package there isn't increased.
I am almost convinced that w/o Ebay, for instance, the U.S. Postal Service wouldn't exist. They have some unbelievably low rates, and they offer services, such as Media Mail and Parcel Post, and when comparing to Fed Ex or U.P.S., it is almost a "no-brainer". I just wish that the Postal Service had a better tracking system since I can receive or send something, yet tracking isn't live and can literally take a couple hours before I get a confirmation email response that the "item was delivered".