Has the buyer opened a dispute, or just sent a message? If she opens a dispute through eBay's money back guarantee for item not as described then yes, you have to pay the return postage. If she opens the same dispute through PayPal then she would pay for the return postage.

As you still have low feedback, a defect could affect your seller rating significantly if she opens a dispute. If she hasn't opened a dispute, I personally would offer her the refund plus return postage costs. Tell her to send it back with tracking and when you've received it, you'll give her the refund. Reinforce that you will pay the return costs. Tell her that there is no need to open a return dispute as you are more than happy to deal with it just via eBay messages.

If it was a change of mind return (which it doesn't sound like it is), then she'd have to pay return postage and she would only be entitled to the item price back, no postage.

Do you think you still have the authenticity certificate somewhere at home? If you think you do, see if you can find it before you relist it. There are so many good fakes on eBay (and not so good fakes) that it's best to show proof that it's genuine. Plus, LV often get listings shut down as there is no proof whether they are genuine or not, so they are all treated as being fake.

Edited to add: I was typing too slow Lyndal! SNAP!