In my neighborhood, I frequently walk past “shop local” signs in the windows of struggling stores. Yet I don’t feel guilty ordering most of my family’s household goods on Amazon. In a world of fair competition, there will be winners and losers.

But when a mail truck pulls up filled to the top with Amazon AMZN, -1.78% boxes for my neighbors and me, I do feel some guilt. Like many close observers of the shipping business, I know a secret about the federal government’s relationship with Amazon: The U.S. Postal Service delivers the company’s boxes well below its own costs. Like an accelerant added to a fire, this subsidy is speeding up the collapse of traditional retailers in the U.S. and providing an unfair advantage for Amazon.