well that puts a spanner in the works. It's up to her to make sure she has the correct address in paypal not you. She should have to pay for return postage as she instructed you to send it to a wrong address the first time. The problem is if you send it to that address now if anything should happen you will not be covered by paypal if she makes a claim. The trick is trying to get her to pay for postage again without making her narky so you get a neg or anything.
If you refund her payment. And she goes into her paypal account and changes the address then she repays you will be covered. explain to her you are require to send all parcels to the address in paypal to be covered should anything happen. With the cost perhaps suggest you pay half each?