Lately, when he isn’t trying to blame China on America’s competitiveness woes, President Donald Trump has become obsessed with the online retailer Amazon. While there’s speculation that Trump is using the reins of government to carry out a personal grudge because Jeff Bezos, Amazon CEO, also owns The Washington Post, the more recent obsession is based on his belief that the United States Postal Service is subsidizing Amazon’s activity. The claim is that, based on a cost-plus method of pricing, Amazon is being subsidized $1.47 per package delivered by the USPS as a last-mile carrier. With an...