You know you have made an error by refunding the buyer before getting your item back. Ebay don't have any mechanism to reverse a refund - they can't 'dip into' buyers' bank accounts in the same way they can sellers.
Be pleasant and polite to your buyer - they can't reasonably expect to keep the item and the money.
You may be able to send them a label, see below, if not, you can purchase a pre-paid Special Delivery envelope from your Post Office and send that to them inside another envelope.
(Edit) If you do get the set back, I would suggest you don't relist it on Ebay. They don't allow fakes and the marks on those items don't look genuine.
Upload your own label
When you're responsible for return postage costs, the primary option available to you is to provide your own return label from a carrier of your choice.
When you use this option, make sure to upload the label and tracking details to the buyer's return request. For items that have a total cost of £450 or more, signature confirmation is required.
How to upload your own label
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)
