I see some ways this could be exploited.
Let's say I have a nemesis, I could in theory spend 100$ in packages from China, and ship it to them. And they'll have to pay 245$ in tariffs ? (245% today).
It's not exploitable in that way.
The bill received is a "you must pay this if you want the package. If you do not, we will destroy the package". It's not a contractual obligation where you get sent to collections or take a credit hit if you don't pay.
In the situation you described, the end result is that your nemesis does nothing, pays nothing, and you are out $100.