As much as it pains to do it, now you're stuck and have to take it to the post office, wait in line and show it to the postal clerk explain to him/her and say that you've mailed these several times before with postage stamps on them, it doesn't weigh more than 13 oz, so why did this get returned to me? Let them explain it to you. Perhaps it was just an accident. We're human beings (and not perfect) so by chance that's what happened here. I'm always willing to give them the benefit of the doubt (the first time). Perhaps there's a new policy in place or the price of postage has gone up?

Good luck sending that out - do let us know what they said about it.. we'd love to hear what it was.