USPS provides a different service than FedEx, UPS.
USPS by mandate had to provide daily mail service to everyone, everyone in the U.S., irrespective of profit. I live on a farm 10 miles outside of town, and USPS had to send a mail truck out to my house every day. They sure as shit were not making money on that deal. Every small town in America has a post office, irrespective of cost-benefit or profit.
In short, the USPS provides a service society deems necessary and desirable outside the constraints of profit and raw capitalism.
Fed Ex is providing a specialized as-needed service for an individual fee. Different service than USPS. Fed Ex could never make money doing what the Post Office does - delivering mail every day to everyone, everywhere in the United States, and keeping Offices open in every po-dunk town and village.
With regard to agriculture, allow me to say "you didn't build that". American agriculture depends on massive government subsidies in research, water projects and irrigation, crop science, price supports, and genetic science. In that sense, our agricultural production is a hybrid system, and is most certainly not pure, raw capitalism.