A Citigroup analysis finds each box gets a $1.46 subsidy. It’s like a gift card from Uncle Sam. 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 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...