When items weigh more than 11 oz. the first class postage becomes Priority automatically. At that weight you have a choice of Priority or Parcel Post. Either of these options incurs a different cost for each different delivery zone. So a regular Priority package from New York to California crosses about 5-6 zones and is more expensive than a Priority package to a nearby zone. Using the flat rate Priority products not only saves you money when the item is heavy, it eliminates the extra zone charge. It enables mailers to always know how much their postage charge will be without having to use a zone chart. It also allows the Post Office to know that the postage is correct without having to weigh each item and check the from and to zip codes.
By the way, FedEx is cheap only for organizations who have a contract and account with them. If you don't work for someone with a discount and you send FedEx, you will pay consideably more. At least the Post Office charges pretty much the same for packages to anyone who shows up to mail!