I would get the buyer to open a Not received dispute then once they have refund.
Have you put the address onto RM to see if you can see the address there.
If you return it then the buyer should pay but if they have not received it the first time then it could happen again. They also could open a Not received dispute and win on the first delivery.
They could buy again if you relist.
If you do send it I would use Royal Mail
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