(declaimer: I am from Hong Kong, and have worked on some import tax/declaration system)

Import tax and tariff relies on clear and accurate import declaration.

For DHL/FexEx imported package, you can fine them when the declaration isn't right.

For USPS imported package, you can't fine USPS -- they didn't know what's in package when it arrive. You can't fine Hong Kong Post or China Post, because they are not US entities. Rejecting parcel in bulk is one of the reasonable option if they can't get the shipper fill in the correct declaration form.

>For USPS imported package, you can't fine USPS

Or you could do what pretty much every other country in the world does, and fine the recipient.