I'd message the buyer and say I'm happy to refund on return, but since it's still in transit, I can't action anything on the request just yet - ask them to RTS the package, and hopefully they will be happy to follow through with that.

I would be reluctant to accept or refund before they've even received - as then they can just keep the item for free, or cost you postage unnecessarily, and their actions were ignorant and careless at best, intentional at worst.