How did the customer pay originally?
If you go to your My eBay-> Selling-> Order Details-> All Orders you can see the purchase.
Now scroll down the left hand Action column to View Order Details and there is the address you shipped to.
Since you are new, you may be able to get your money back - maybe, perhaps, possibly- by going to the eBay employees on social media.
https://community.ebay.com/t5/Selling/How-do-I-contact-Customer-Support/m-p/32016431#M1783851
https://www.facebook.com/eBayForBusiness/ — Message button in upper right on landing page.
https://twitter.com/askebay?lang=en
Write out your problem and proofread it before posting. (Your title on this post is confusing, you can do better).
The staff covering FaceBook have a good reputation.
Explain that you didn't know how to find the address for the return shipping label and ask if this can be corrected.
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Politely- Captain Malcolm Reynolds