It’s the postage that’s counterfeit.
But none of this has anything to do with customs.
Chinese ship to the US in large consolidated containers with thousands of packs from numerous companies.
Once the container is through customs, another “reshipping” company “relabels” the individual packages with US Postal Service postage labels with the final US shipping address on it.
A scam used by these “reshipping” companies is to reuse electronic postage codes by printing them on new labels.
Tens of millions of dollars in postage have been stolen this way.
The problem for recipients is that once they’re seized for counterfeit postage, they can be opened without a search warrant, something not usually allowed.
It’s potentially bad news. Unlike a seizure by customs, which only results in a letter, post office seizures can result in local law enforcement getting involved.