Hopefully, you put your return address on the back of the parcel, and it will be returned to you.
When this happens, cancel the order citing Problem with Buyer's Address. This fully refunds the buyer, and your seller performance is not affected.
Send your buyer a message explaining the above.
Personally, if there is an issue with their address, and they are not very communicative, I would block them in case they buy from you again, and the same issue arises.
Block them here:
https://www.ebay.co.uk/bmgt/BuyerBlock?
@backdoorman411