*ahem*
Florida
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Florida stands out because of a decision in 1978 in
Fiske v Florida by the Florida Supreme Court which ruled that wild-picked psilocybin mushrooms could not be legally considered "containers" of psilocybin and thus would have to be specifically listed in Florida law by the legislature if they meant to make them illegal. Since the Florida legislature has not chosen to change the laws since then, wild Psilocybin mushrooms are presumptively legal to possess in Florida. This does not mean police won't arrest people for possession of even wild mushrooms, but this has been used as a successful defense against such charges.
Taken straight from Erowid...I'd double check your facts.
And, I'm not saying they couldn't tell there were mushrooms going through the mail. I'm saying they couldn't tell if they were LEGAL mushrooms or not.