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 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.

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In 2001 the quantity of first-class mail in the U.S. began to decline thanks to the internet. Today it is down 40% from its peak levels, according to Postal Service data. But though there are fewer letters to put into each mailbox, the Postal Service still visits 150 million residences and businesses daily. With less traditional mail to deliver, the service has filled its spare capacity by delivering more boxes.

Other companies, such as UPS and FedEx , compete with the Postal Service to deliver packages. Lawmakers, to their credit, wanted a level playing field between the post office and its private competitors. The 2006 Postal Accountability and Enhancement Act made it illegal for the Postal Service to price parcel delivery below its cost.

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In 2007 the Postal Service and its regulator determined that, at a minimum, 5.5% of the agency’s fixed costs must be allocated to packages and similar products. A decade later, around 25% of its revenue comes from packages, but their share of fixed costs has not kept pace. First-class mail effectively subsidizes the national network, and the packages get a free ride. An April analysis from Citigroup estimates that if costs were fairly allocated, on average parcels would cost $1.46 more to deliver. It is as if every Amazon box comes with a dollar or two stapled to the packing slip—a gift card from Uncle Sam.

Amazon is big enough to take full advantage of “postal injection,” and that has tipped the scales in the internet giant’s favor. Select high-volume shippers are able to drop off presorted packages at the local Postal Service depot for “last mile” delivery at cut-rate prices. With high volumes and warehouses near the local depots, Amazon enjoys low rates unavailable to its competitors. My analysis of available data suggests that around two-thirds of Amazon’s domestic deliveries are made by the Postal Service. It’s as if Amazon gets a subsidized space on every mail truck.

The US Post Office has been subsidizing package delivery for decades, and it has continued to do so even though this was forbidden by Congress in 2006.

This is not surprising.  It is a feature of the current economy that subsidies (copyright, patent, implicit bailouts, the Pentagon, stadium deals, etc.) are at the core of the businesses of the big players in almost any US industry.

The fact that they are taking from the rest of us for their profits is a major driver of inequality.