Briefly, and once you get past the intro (which takes half the article): UPS and Fed Ex do not deliver directly in rural areas. They deliver to central post offices, which then deliver in rural areas.
This is a subsidy because, as with so many things in the U.S., we subsidize rural service so that these areas will be served. This is why the post office loses money. Because it charges a uniform rate and must serve high cost areas.
and what will happen to these rural areas if we ever eliminate the post office all together?