SINCE Donald Trump became president, economists have been fretting that America is bent on undermining international institutions. The World Trade Organisation is the body most have worried about. But another target has just come into view: the international postal system.

On September 3rd, at a congress in Addis Ababa of the Universal Postal Union (UPU), a little-known UN agency, the Trump administration launched a campaign to reform “terminal dues”. These are the fees a post office pays a foreign mail service to take packages from the airport they land at to the final recipient. Under UPU rules, developing countries currently pay less for the final local delivery of their international mail than developed ones do. Peter Navarro, Mr Trump’s trade adviser, thinks that the Chinese are exploiting these rules to flood America with cheap e-commerce packages. He noted this week in the Financial Times that it costs more to post a package from Los Angeles to New York than it does from Beijing to New York.

The explanation can be found in the UPU’s history, explains Anna Möller Boivie of Copenhagen Economics, a consultancy. After the organisation was formed in 1874, it was agreed that the post offices would do the final deliveries for each other’s international post free of charge. Incoming and outgoing mail volumes tended to be equal in each country and so any losses from the rules were small.

But decolonisation lifted the number of developing countries in the UPU. In 1969 “terminal dues” were introduced to help fund final-delivery costs in poorer places. Richer countries would pay 70-80% of the cost of domestic post to have their items delivered in other countries, but poorer ones would pay just 20-30%.

As e-commerce from China to places such as America has grown, so have demands to reform the system. According to the United States Postal Service (USPS), losses from inbound mail more than doubled, to $135m, between 2012 and 2016 (though these accounted for only around 5% of its total losses last year). America has natural allies on this subject, points out Jim Campbell, an expert on international postal law. Canada, Ireland and the Nordic countries, as net importers of foreign e-commerce packages, also moan about the increasing cost of the rules.

Mr Navarro wants terminal-dues rules reformed so that the USPS charges the same for domestic and foreign postal items. But the UPU’s one-country, one-vote system means that poorer countries are likely to block that. So Mr Navarro has threatened to rip up the UPU’s rules and have the USPS charge its own rates. No country has broken the regulations in this way before; the one that has come closest is Russia, which bent the rules last year by charging extra for post arriving into the country. To preserve some appearance of obeying the UPU, it does deliver at terminal-fee rates if mail arrives through Mirny, a small town in Siberia. If America does break ranks, that could cause a tit-for-tat war on delivery charges as other countries retaliate, says David Jinks of ParcelHero, a postal broker.

Some e-commerce merchants in America are ready to take the risk. One is Jayme Smaldone, inventor of the Mighty Mug, which makes liquids hard to spill. He noticed a fake Mighty Mug on sale for $5.69, with free shipping for the 8,100-mile journey from China. The cost for him to send a Mighty Mug within America with the USPS was $6.30. Mr Smaldone eventually twigged that China Post was paying the USPS $1.40 for local delivery. “It’s hard to compete against that,” he laments.