There's a line that might open you up to being sued over it. Ridiculous as that would be, I wouldn't really be surprised if the right lawyer in the right court could pull it off.
There's already the precedent from Katko v. Briney[1] that establishes a person can't use deadly force to defend property. Since courts prioritize the avoidance of bodily injury over property it's definitely not much of a leap for a lawyer to argue that a thief who suffered any bodily injury at all from a boobytrapped package has a case.
Whether a prosecutor and/or a jury would be interested in that argument is another story.
Sure. It's all hypothetical, of course. Though, hypothetically, I might also thank them for intercepting a booby-trapped package that was meant for me.
