That's not what happens in the USPS. I've imported two kits of Chinese manufacture from outside China and no tariff has been applied. Until Aug 29th (except for shipment originating from China) a shipment valued over $800 (the De Minimus limit in the USPS) will have to pay tariffs. Shipments over $2500 value have to go through a Formal Customs Clearance and generally a licensed customs broker is required to do that.

What happens after Aug 29th? For the next six months shipments will be assessed a flat fee based on what tariff our government has applied. For example a shipment originating from the European Union (15% tariff rate) will be assessed a flat fee of $80. A shipment originating from Canada (35% tariff rate) will be assessed a flat fee of $200. If the value exceeds $2500 it has to go through a formal customs clearance. Where the product is manufactured below that $2500 threshold is of no matter. The USPS isn't going to start opening packages to see where it was manufactured. They don't have the ability to do that.

Apparently after six months all shipments arriving into the US by postal service will go through the normal Informal/Formal clearance process and will be liable for whatever the tariff is going to be. Better for most modelers than paying a flat fee of $80 to $200.

Also be aware that common carriers such as UPS, FedEx, DHL, etc. no longer follow the De Minimus rules. Today they will charge you a clearance fee (often noted as a government fee) and a tariff rate. Both are based on the value of the goods. A friend recently ordered about $1200 in goods from the Czech Republic and paid about $333 in clearance fees and tariffs.