The US Postal Service (USPS) said it has stopped accepting parcels from mainland China and Hong Kong until further notice.
Letters will not be affected by the suspension, said the company, which declined to offer a reason for the decision.
However, on Tuesday new rules came into force which closed a loophole that allowed small packages worth $800 (£641) or less to be sent to the US without paying tax or fees.
It was one of the measures announced by US President Donald Trump who imposed an additional 10% tariff on all goods imported to the US from China.
The so-called de minimis tax loophole has faced increased scrutiny in recent years as Chinese e-commerce giants like Shein and Temu have used it to reach millions of US customers.
Changes to the tax exemption under President Joe Biden were already in progress before Trump took office.
But in his trade announcement at the weekend, Trump extended tariffs to all China goods being imported into the US, including fashion items and toys.
In response, China said it would implement tariffs on some US imports.
From 10 February coal and liquefied natural gas (LNG) will face a 15% levy. Crude oil, agricultural machinery and large-engine cars will be subject to a 10% tariff.
Nick Stowe, chief executive of Monsoon & Accessorize, told the BBC’s Today programme he was in support of the changes in the US to the de minimis exemption, claiming that it had allowed major Chinese retailers to “undercut” rivals in other markets.
“It has long been a complaint of UK retailers, European retailers as well as the US retailers that Shein are exploiting this loophole, not paying customs duty and they have built a business at an industrial scale,” he said.
Trump had been expected to speak to his Chinese counterpart Xi Jinping in the coming days but on Tuesday the US president said he was in “no rush” to hold talks.
“Trump’s tariff changes are especially sharp if goods were previously shipped via e-commerce directly from China to the US,” said trade expert Deborah Elms.
Close to half of all parcels entering the US under de minimis exemptions were sent from China, according to a 2023 report by US Congress.
Officials have pointed out that the large flow of parcels entering the country through this exemption made it increasingly difficult to screen them for possible illegal goods.
The BBC has contacted USPS to request more details about the decision.
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