Actually this is easily done. We moved 3 weeks ago and I changed my address on both Ebay and Paypal, but for some reason it keep reverting back to the old address.
I found this out when I bought some trousers, I clicked through all the payment details etc and only really noticed the address was wrong when I was paying for something else.
I contacted the seller straightaway and luckily she hadn't posted it yet.
So the buyer isn't stupid at all. She may well have changed her address like I did, but Paypal have not registered it, could be a fault. I would suggest she contacts the Post Office and ask them to redeliver, or contact the people at that address and ask them to send it on.
Not your fault, but not really hers either.