Please keep abreast of the de minimis rules, because some of them state that the importer may pay between $80 to $200 per item. I'm not sure if "an item" refers to the parcel as a whole or to every individual thing inside the package. Alternatively some packages will pay a percentage (the tariff amount?) instead. However from what I've read you, the importer, do not get to choose which you will pay. This applies to all countries, not just China. This rule went into effect on August 29, 2025.
The flat fee option is available for 6 months only, for shipments where it would save money vs. a percentage.
It will most likely be used by parcel aggregators consolidating many individual shipments from an overseas hub to a US hub, or by importers with warehouses on both shores. In many cases that is what you will be dealing with, buying imports. In cases where you're not, like certain sellers in Japan who insist on doing everything the hard way, you will be paying a percentage unless your order is Wicked Huge.
The most likely scenario to see the flat fee assessed, when dealing with a single -average sized- international shipment, is if the shipper lowballs the value and gets caught.
If this happens to you, simple solution. Don't pay it. The package will get stopped by customs. File a charge back if you must, to recover your initial price. The shipper won't make that mistake again, believe it.
If a shipper charges you $80 to $200 claiming it's a customs duty, don't pay it, because it's potential fraud. Shippers are not authorized to collect duties. I don't even know what amount people pay to collect the exports I ship. Except when they gripe that I didn't lowball the value. (See above)