“The United States Postal Service does not directly own or operate any aircraft or trains, although both were formerly operated. The mail and packages are flown on airlines with which the Postal Service has a contractual agreement. The contracts change periodically. Depending on the contract, aircraft may be painted with the USPS paint scheme. Contract airlines have included: UPS, Emery Worldwide, Ryan International Airlines, FedEx Express, American Airlines, United Airlines, and Express One International.”
Basically FedEx is getting revenue via contract running stuff operated by FedEx and other companies.