Repeated had sold item returned to me

[doomdoors]

I've had a tiny cheap item (large letter size) returned to me twice by Royalmail because the address doesn't exist.
(I didn't check in between because the 1st time it didn't state that it was mid-addressed and I use RM click and collect so it's all automated as it's linked to eBay).
I'm a private seller, just selling odds and sods from a de~clutter.
If I send it again I will have spent more money on postage than the item!
What am I expected to do in this case?

Keep it.
Check the address online
Message the buyer.

Just message the buyer?

I've messaged the buyer, the address is correct and there's a letter box (it wasn't signed for extra and would fit through a letterbox easily).

It's a shame, but I'd refund the buyer on the grounds there is a problem with the address, and relist it.