Beginning Wednesday, shipments arriving in the United States from China and Hong Kong worth less than $800 could face 54 percent tariffs.
Earlier this month, President Trump closed a longstanding loophole that had allowed a flood of inexpensive Chinese goods to be mailed to the United States without any tariffs.
Starting on May 2, those packages faced a tariff of 120 percent or a $100 flat fee.
After the United States and China agreed this week to a temporary truce in trade tensions, that tariff is now 54 percent. The changes, which took effect on Wednesday, were described in a White House executive order and guidance from Customs and Border Protection.
For the past decade, a tax loophole known as the de minimis exemption allowed goods worth up to $800 to enter the United States without import duties. The result was millions of packages shipped from China to the United States, as American shoppers got hooked on buying everything from flash drives to water bottles at low prices.
Chinese companies like Shein and Temu built their businesses around the loophole, sending goods made in Chinese factories directly to American shoppers. At the same time, China pushed its manufacturers to find buyers overseas.
Last year, nearly four million packages a day entered the United States with no customs inspection and no duties paid, angering American businesses that said the loophole made it difficult for them to compete.
Mr. Trump said the loophole had created a pathway for the chemicals involved in making fentanyl to come into the United States from China because of limited checks on these packages.