Well, I'm not a customs agent, but I'd imagine they do it in the same way they adjudicate anything else: inspection. Some things get through by chance, of course, but not at a rate you'd want to rely on if you're a business.

In particular, if I walk into a random post office and send a one-off shipment internationally, the paperwork, origin, packaging, manifest, etc. is vastly different than what, say, Temu was doing to ship a $10 widget to US consumers at scale.

The rule you're talking about is not new, so presumably they've figured it out.

If you're saying post offices around the world are wrong, it might be time to reevaluate your own statement for truthiness.

There's multiple countries that are now suspending shipments over $100 to the US. So either there is a huge fuckup in communications from the US to every other country or there's a fuckup in the process itself.

None of them wants to have a whole bunch of consumer/small business shipments stuck in US customs for who knows how long it will take for the US to figure out exactly what tariffs it wishes to charge and how exactly it plans to collect them, so are leaving it to the higher-priced experts like DHL (who will only do it if you’re willing to pay for their Express service, not their Standard parcel service from Germany), UPS, or FedEx.

I doubt they’re conspiring to leave money on the table just to make Trump look bad.