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.Emailing the person in question, he believes me that the envelope was empty and says now that the lady at the post office was in a hurry and he didn't wait to make sure that she actually sealed the cash inside.
Hm.
As for splitting the loss - I'm just doing what I hope someone would do for me. Then again, I think I would have packaged the cash myself and never let a postal employee do so! Ah well, this has been a month of losses, and a month of lessons!
But yeah this person, based upon your posts after this prove he/she is likely a scammer.
I wish you the best of luck with your business as you seem highly reliable and everyone is saying youre good!