The $100 rule might not be new, but given that it was by far exceeded by the $800 de minimis exemption until now, it just didn’t matter.
This has nothing to do with the value threshold. US customs had to know the difference between personal packages and commercial packages before the change.
You asked me what distinguishes a commercial package from a personal gift.
> US customs had to know the difference between personal packages and commercial packages before the change.
Presumably for things like import restrictions (I could imagine somebody sending homemade cookies is treated differently than a large-scale food importer), but not for a decision on whether to charge or not levy duties though, right?
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.