I would suggest calling paypal and discussing it with them and see what they suggest. Sadly because it was sent surface, it's going to take forever to get there, It probably wont update until it reaches the destination country.
In your correspondence to the buyer via paypal, you could write something like this:
Spoiler
Dear Buyer,
I can understand your frustration and concern that your package has yet to arrive. Rest assured your parcel was sent out on _________ and the tracking number is ____________, as you know surface mail can take up to 6+ weeks. I would appreciate it if you could kindly wait another 10 days for the parcel to arrive, if it hasn't arrived by _____________, I will refund your money.
The thing is, if you provide the tracking and it still doesn't show anything, paypal will side with her, so you're better off trying to remain civil and understanding, keeping in mind how you would feel if roles were reversed. If she has until the 16th to escalate it, explain that you understand that she opened a case to protect herself and her money (understandable), and then just ask her to please give it some more time.
That's what I would do anyways.