(h) For avoidance of doubt, duty-free de minimis treatment under 19 U.S.C. 1321 shall not be available for the articles described in subsection (a) and subsection (b) of this section.
What is that?
Ever order something off Alibaba, eBay or Amazon -- that is shipped in from China?
Its been exempt from import duties because of that rule. Essentially "small shipments"
This has been wildly exploited. The original law is why you can bring a small amount of personal purchases into the United States without paying duty on them. But this was expanded and has been wildly exploited for years -- the current limit, for example is $800 per shipment from China and it does not have to accompany you on the way in.
So various selling channels are set up so that each shipment to a person is under that amount, and thus there is no duty even when there would be otherwise. The intent is to let small sellers -- and travelers -- come into the United States without paying duties. It was never intended to, but has been exploited to, permit organized trade via sites like eBay, Alibaba and Amazon in the billions from China into the Unites States without paying the duties that would be payable had a retailer, say, WalMart sourced the same products for retail sale to Americans.
It is facially reasonable to let people travel and, as a small percentage of their total expenditures buy things in that nation and import them back into the US without having to keep their receipts for everything and then calculate a duty on it. That makes sense. But this has been wildly abused by various firms and neither Congress or the Executive has been willing to consider "bundling" (which is basically what they're doing) a circumvention and thus levy duty on the aggregate amount that comes into the US on a per-month basis irrespective of the number of shipments when coordinated by a single selling or marketplace entity. That too is reasonable.
Well, Trump just slammed the door on that game at least in respect to Canada, Mexico and........ the big one..... China.