I have not generally been in the business of defending Amazon, but I thought I would throw in a word or two of clarification around Donald Trump's claim that the U.S. Postal Service is "dumber and poorer" because of its deal with Amazon. Trump's claim is based on a Citigroup study that found that Postal Service loses an average of $1.46 on each package it ships for Amazon. The Postal Service claims that it profits from its arrangement with Amazon and that it would lose business if it raises its rates.

There actually is a very simple explanation for the differing assessments. The Postal Service has a huge amount of fixed costs in the form of retiree benefits and especially retiree health benefits. Congress has required that the Postal Service pre-fund 75 years of retiree health benefits. This requirement sets the Postal Service apart from private businesses, who do little or no pre-funding of retiree health benefits. It also accounts for almost all of the Postal Service's losses over the last decade.

But the accounting issue is independent of this requirement imposed by Congress. Essentially what the Citigroup study did was impute the largely fixed cost of retiree health benefits to the various sections of the Postal Service's business. If these costs are imputed to its delivery of packages for Amazon, the Citigroup study finds they are coming up short by $1.46 a package.

But this is just bad economics. The question for the Postal Service is whether it is recovering its marginal costs — the additional amount spent on labor, gas, wear and tear on vehicles, etc. — with the prices it is charging Amazon. The Postal Service claims it does (I have not tried to check their calculations), and if that is true, the Postal Service is coming out ahead from its deal with Amazon.

So the loss claimed by the Citigroup study is clearly wrong and Donald Trump is wrong to be using it to attack the Postal Service, Amazon, and Jeff Bezos. On the other hand, Amazon has gotten a subsidy worth tens of billions of dollars since its creation as a result of not being required to collect sales taxes in most states for most of its existence. This subsidy almost certainly exceeds its cumulative profits since it was created, so people do have serious cause to complain about Amazon.