I like how @jbealsmusic
has a warning on products
on the Next Gen Guitars site for products that should not be shipped in the winter. I don't think it should be required, but it's a smart move to combat some people's questionable decisions. Shipping guitars in the wintertime probably should have the same warning, especially if going cross-country where several different temperatures could be experienced. Please note, I'm not blaming the OP for not providing such a warning. It would help to ask the buyer, "Are you sure you want me to ship this now?" But at the same time, the buyer should bear responsibility for seeking out an item that needs to be shipped a long distance in the cold.
One question is should the buyer have known better? Is this their first nitro guitar, or are they a collector who has purchased lots of similar guitars? If it's some kid who didn't know better and was so excited to look at their new guitar and they didn't know to wait, that might sway your decision making compared to someone who has done this several times before.
I presume the OP is not a shop, and so the buyer should expect to assume the risk for choosing to get a deal by purchasing from a distance rather than buying locally. If you wanted to "go above and beyond", try to determine how much the checking has affected the value and offer to refund half of the difference. Clearly the buyer should assume at least half of the responsibility for the issue as they did not wait several hours to allow the package to acclimate. If they don't want to go that route, allow them to ship it back to you (at their expense) and refund the purchase price minus half of the difference in value as a "restocking fee" when the guitar arrives back in good condition (you need to incentivize the buyer to do a good packing job and adequately insure the shipment). As a private transaction, I think both of those options go far beyond what a typical Kijiji purchaser should expect.