Kim Batten, a physical therapist from Oakland, Calif., bought a trench coat earlier this year from a Dutch retailer for $456, a price that was a little above her budget.

But after the coat was shipped, UPS told Ms. Batten in an email that she would have to pay over $250 in customs duties to receive it. With shipping, the whole transaction came to well over $700.

“It ended up being the second most expensive article of clothing I’ve ever bought, other than my wedding gown,” she said.

For decades, no customs duties were imposed on items like Ms. Batten’s coat. But this year, President Trump closed a loophole that had allowed goods worth $800 or less to enter the United States tariff free. The loophole, known as the de minimis exemption, ended for items from China in May and for the rest of the world in August. Shoppers must now pay duties for the first time, often in amounts far higher than they expected.

“I’m definitely more leery about where I’m shopping now,” Ms. Batten, 35, said.

Democrats and Republicans favored ending the exemption on low-value goods. They said it helped large sellers, particularly those from China, evade tariffs, and made it easier to smuggle fentanyl into the country. Other countries are now planning to end or tighten de minimis exemptions.

Still, the abruptness with which the United States closed the loophole caused major disruptions for sellers of all sizes, as well as for express carriers like FedEx and UPS, postal networks and American shoppers.

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