It could be the truth though. Maybe someone is stealing their mail.
I am happy to hear paypal paid them and it didn't come out of your pocket. I wonder if they do the same if the item is damaged or lost. Because they really ought to protect the sellers that way.
Also, opening a case might have been the only way for you both to win in this situation. Say the item was really stolen so she lost out and wants her money back, and you sent it so you don't want to just refund her and lose out on the item and money, then by her opening a case you got to keep the payment and she got a refund from paypal. Although it doesn't help you with the extra cost of re-shipping.
Anyway, my point was that opening a case could be good for both sides assuming you both did everything right and it is neither person's fault and assuming paypal really does protect the seller and buyer.