Less than a year ago, executives from FedEx and UPS were talking about how they were handling a flood of packages from China to American consumers.

“Explosive” is how Carol Tomé, UPS’s chief executive, in July described the volume of shipments from e-commerce companies selling Chinese goods in the United States. And FedEx’s chief customer officer, Brie Carere, said about those companies in June, “No one carrier can serve their entire needs.”

But that torrent is expected to slow to a trickle after President Trump on Friday closed a loophole that had allowed cheap goods from China to enter the United States without paying tariffs.

The business of transporting hundreds of millions of low-value shipments on as many as 60 freighter flights a day between China and the United States could now wither.

A falloff in such shipments could deprive companies like UPS, FedEx and DHL of a big source of revenue. Airlines, mainly those that carry only cargo, and smaller logistics companies could also suffer. Passenger airlines may also be hurt somewhat because they carry some of those packages, too.

UPS said last week that it expected the revenue from shipping packages from China to the United States — its most profitable trade lane — to decline roughly 25 percent in the second quarter of this year, from a year earlier. UPS also announced that it would cut 20,000 jobs this year as part of a long-term plan to reduce costs, and said “macroeconomic uncertainty” prevented it from updating its forecasts for revenue and profits for 2025.

Ms. Tomé said UPS’s China-to-U.S. business was responsible for 11 percent of the company’s international revenue. She suggested that the company could take the trade tensions in stride, saying that, when trade between China and the United States declined during Mr. Trump’s first term, it increased between China and rest of the world.

But because Mr. Trump is now waging a more aggressive and broader trade war, logistics companies may not be able to easily make up for lost sales in other places, as they were able to during his first term, analysts said.

“It was a bit of a bumpy ride the last time,” said Jay Cushing, an analyst for Gimme Credit. “It took a little while for things to level out, but this is probably going to take even longer.”

The tariffs that Mr. Trump imposed on Chinese goods during his first term helped set off the gusher of inexpensive goods from China.

To avoid those tariffs, Chinese sellers increasingly sent products to the United States under the loophole that was closed on Friday for imports from mainland China and Hong Kong.

Known as the de minimis exemption, the loophole allowed buyers to import goods worth $800 or less without paying tariffs or filling out detailed customs paperwork. Now that the exemption is gone, American shoppers will have to pay tariffs of as much as 145 percent on Chinese goods, adding $14.50 to the cost of a $10 T-shirt.

Temu, one of the biggest e-commerce companies selling Chinese goods, said last week that it was no longer shipping orders from China directly to American consumers. “All sales in the U.S. are now handled by locally based sellers, with orders fulfilled from within the country,” Temu said in a statement.

As the ending of the exemption loomed, Wall Street analysts pressed delivery companies to predict the impact.

When asked on an investor call in March what share of revenue came from de minimis shipments, FedEx’s chief executive, Raj Subramaniam, said it was a “minority.”

Isabel Rollison, a FedEx spokeswoman, declined to offer a more precise estimate. “In terms of our revenue split by geography, we serve an extremely diversified customer base across more than 220 countries and territories,” she said in a statement.

DHL, based in Bonn, Germany, also declined to say to say what percentage of its business came from de minimis shipments from China. Glennah Ivey-Walker, a DHL spokeswoman, said they represented “only a small portion of our overall U.S.-bound volume and our overall business volume in the U.S. market.”

Ending the exemption might have been worse for the carriers had it not been for a late change to the rules by the Trump administration.

The lower-value goods were set to become subject to strict customs rules that require detailed paperwork. But the administration late last month issued a waiver that allowed the goods to be treated more leniently.

Some trade experts said the administration’s change undermined tariff collection because it deprived Customs and Border Protection of information it needed to make sure that importers were paying the correct amount of import duties.

“If you don’t know exactly what the good is, it’s hard to know what the right potential value is or what the right tariff should be,” said Lori Wallach, director of a trade program at American Economic Liberties Project, an organization that seeks to curb the power of large corporations.

But some customs lawyers said that, even after the waiver, detailed information would still be required.

The waiver came after DHL stopped making some shipments that were subject to the paperwork requirement, and after it had spoken to members of the Trump administration.

Ms. Ivey-Walker, the DHL spokeswoman, said the waiver would not “make it harder to collect tariffs or in any way impede the government’s ongoing efforts to protect its borders.” She added that DHL had spoken to the administration to highlight the delays that might occur if the detailed paperwork requirement was enforced.

A sharp decline in low-value shipments could also shake airlines.

Air cargo shipments had already slowed even before the end of the exemption on Friday.

By mid-April, air cargo traffic from mainland China and Hong Kong to the United States was down about 16 percent from a year earlier, according to WorldACD, an industry data firm. And experts say that traffic is likely to slow further in the coming weeks.

“We expect to see as much as 30 to 40 percent of China-to-U.S. capacity come out of the market,” said Derek Lossing, the founder of Cirrus Global Advisors, an e-commerce and supply chain consulting firm.

The carriers most active in e-commerce trade between China and the United States include two U.S. cargo airline companies, Atlas Air Worldwide and Kalitta Air; Hong Kong’s Cathay Pacific Airways; and the cargo divisions of Chinese airlines, according to several air cargo experts.

U.S. passenger airlines are not as vulnerable because they operate relatively few flights between the United States and mainland China and Hong Kong.

To make up for the losses, Chinese businesses may try to sell more goods to customers elsewhere, including in Europe, Australia, New Zealand and Latin America, experts said.

There are already signs of such a shift. While air cargo shipments from China to the United States were down in the weeks leading up to the expiration of the exemption, flights into Miami, a hub for flights to Latin America, were up slightly, according to Mr. Lossing.