Improbable have got a nice article up at the moment - the results of them posting various items, some strange ones, with the US postal service.
Here's a snippet: Helium balloon. The balloon was attached to a weight. The address was written on the balloon with magic marker; no postage was affixed. Our operative argued strongly that he should be charged a negative postage and refunded the postal fees, because the transport airplane would actually be lighter as a result of our postal item. This line of reasoning merely received a laugh from the clerk. The balloon was refused; reasons given: transportation of helium, not wrapped.