For more than a century, postal services in various countries have, through the Universal Postal Union, agreed to deliver mail that originated in another country. This service used to be free, until a 1969 update required that postal services pay one another “terminal dues”—fees for delivering another country’s mail—based on how developed a country was: Countries whose postal services were still in transition could charge high dues, while developed countries like the United States would have to charge low dues. In 2006, a new law allowed the United States to enter into bilateral agreements with foreign posts, and essentially agree on terminal dues on their own.

In 2011, when e-commerce really started taking off in the United States, the U.S. Postal Service entered into a bilateral agreement with China Post that gave sellers first-class tracking and delivery confirmation for very low rates as long as an item was an “ePacket” product, weighing less than 4.4 pounds. With shipping so cheap, and manufacturing in China already so inexpensive, the goods started flowing. The volume of ePackets more than doubled from 2014 to 2016, according to the Postal Service. This helped bring in about $493 million in revenue for the USPS, but also added some costs. Specifically, because it’s so expensive to send a product to China, packages that had to be sent back because they were undeliverable cost the USPS anywhere from 20 cents to 57 cents each.

That wasn’t the only problem created by the flood of goods from China. Much of the fentanyl currently circulating in the United States has come from online sellers in China. Shipping small electronics and cosmetics across the sea is bad for the environment: One container ship causes as much pollution as 50 million cars. And many of the goods making their way here are cheap knockoffs of products made in the United States. But they’re inexpensive, so Americans keep buying them.

Read: The problem with buying cheap stuff online

Wednesday’s White House announcement means the United States will soon adopt its own self-declared terminal dues. The United States will also file a notice that it will withdraw from the Universal Postal Union, the White House said, though if it can negotiate new agreements with other countries, it will not withdraw.

The announcement is quintessential Trump—it shatters decades-old respected treaties and threatens to withdraw from an international consortium unless other countries accede to the wishes of the United States. But it would be a boon for the U.S. small businesses competing with Chinese sellers on sites like eBay and Amazon, as well as the brick-and-mortar retailers trying to keep up with online sellers who sell cheap goods from overseas. “U.S. Amazon sellers will no longer need to compete so narrowly on price due to the level playing field,” said Chris McCabe, who for six years worked for Amazon, reviewing and suspending seller accounts, and now runs a seller consultancy, ecommerceChris.com.