That is a different story again to being caught with them red handed.. From my experience if your parcel gets seized, US customs will send a letter to the delivery address on the parcel informing you that they have seized the item, and ask you if you want to take responsibility of it.. If you don't respond to the letter they destroy the item after 30 days, but if you sign the form you are confirming they are infact your banned substance and will go from there, take you to court or what not.
They cant do much if you don't take responsibility of it unless of course it happens on a common occurrence lol..