If you really wanna be raunchy you could always send back some used toilet paper, although I'm not sure if there would be any legal repercussions to that. I would imagine the person who sends fecal matter in the mail isn't too concerned with legal repercussions, though. Plus, you could always deny it, maybe pretend like a postal employee did it. I'm sure that would be believable with what congress is proposing to do to the post office in the near future.