Rows of white concrete buildings near the Pearl River in southern China house one of the world’s fastest-growing industries: Gritty workshops are churning out inexpensive clothing that is exported straight to homes and small businesses around the world. No tariffs are paid, and no customs inspections are conducted.

       The laborers who make these goods earn as little as $5 an hour, including overtime, for workdays that can last 10 hours or more. They pay $130 a month to sleep on bunk beds in tiny rooms above factories packed with sewing machines and mounds of cloth.

       “It’s hard work,” said Wu Hua, who sews pants, seven days a week, at a factory in Guangzhou, a vast metropolis that straddles the Pearl River.

       E-commerce giants have forged close links from international markets to workers like Mr. Wu, shaking retailing and economies around the globe.

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       The number of duty-free shipments to the United States has risen more than tenfold since 2016, to four million parcels per day last year. Similar shipments to the European Union have climbed even faster, reaching 12 million parcels a day last year. Duty-free shipments to developing countries like Thailand and South Africa have also surged.

       Now a global backlash is underway. President Trump ordered a halt on Feb. 4 to the duty-free entry, without inspection, of parcels with goods worth up to $800. Mr. Trump temporarily suspended his order to give officials time to devise a plan for dealing with the mounds of parcels that immediately started piling up at airports for inspection.

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