Ebay: Sent an item to buyer in Canada, lost in the mail?

I sell a few items every month on eBay, and recently sold a sweater to a buyer in Canada. Unless the buyer pays for Priority shipping, there is no option for tracking packages to Canada USPS, and I inform all buyers of this before shipping to them. Unfortunately, a package I shipped a month ago has still not arrived, and the last info on the customs form shows it going over the border, where tracking info stops. Nevertheless, despite having sent her the full disclosure email before shipping, and confirming her shipping preference without tracking, the buyer is asking for a full refund, which would pretty much wipe out my profits for this month, and then some, on eBay. What should I do in this situation?