Today we received 3 cash orders ... one of which was an empty envelope. It was not tampered with, nor was the label removed and replaced, so either someone stole the cash from the package miraculously, or the client sent an empty envelope knowing we would at least split the "loss" with them. We did split the loss and sent $400 worth of btc, but this makes it clear that accepting payment via this method is not a reliable option. 
It's very admirable of you to have sent the $400 dollars worth of btc to whomever anyway. But I suggest you make people understand that if money is missing from the envelope you are not responsible. As you shouldn't be.