I would do nothing further without asking the buyer to write down their address in a message to you.

For all you know, they might have missed a number or something.

Once you establish what was wrong with the address, then you can decide what to do.

If the buyer inputted their info incorrectly, then they are responsible for another round of postage. 

You can, of course, cancel the transaction, but remove the postage from the refund as it has been used.

Just see what happens first with what the buyer writes in their confirmation of address.

I had a transaction once where the address was literally to a construction site, but the buyer insisted it was correct.