The Washington Post headlined this “The Postal Service is losing millions a year to help you buy cheap stuff from China“:
This strange consequence of postal law was less significant when the mail was mostly personal correspondence. But as Chinese companies began logging on to Web marketplaces like eBay, Amazon, and Alibaba, they started taking advantage of the shipping deal to sell directly to American consumers. And so it’s never been easier to get something cheap and Chinese delivered to your door for a startlingly low price: $4.64 for a digital alarm clock; $2.50 for a folding knife; $1.88 for an iPhone cable — all with shipping included…
Countries used to provide this forwarding service to each other for free, but in 1969 an update to this postal treaty called for small fees (called terminal dues) on each mail piece. Since then the dues have grown, and the payment system has become labyrinthine. In most cases, however, postal services still charge each other less than they would charge their own citizens for moving a package across the country.
According to the terms set out in Universal Postal Union treaty, the USPS in 2014 gets paid no more than about $1.50 for delivering a one-pound package from a foreign carrier, which makes it hard to cover costs.  The USPS inspector general’s office estimated that the USPS lost $79 million in fiscal year 2013 delivering this foreign treaty mail. (The Postal Service itself declined to provide specific figures.) …
At the latest round of negotiations in 2012, countries did agree to raise fees slightly. The United States will get to charge about 13 percent more every year from 2014 to 2017. Under the new terms, the inspector general’s office believes that the USPS will start to lose less money on inbound mail. 
All this should be a reminder that any trade deal has winners and losers and unintended consequences. Internet commerce suddenly made the terms of a long-standing mail treaty a competitive advantage for Chinese merchants, and U.S. importers like the McGraths have been feeling the squeeze. But this same system also means that average Americans can get a really sweet deal on an iPhone case shaped like an Absolut bottle.
Hat tip to Corinne Low.