Sorry to hear that. I'm guessing the buyer either didn't read the postage part of the listing, or assumed the item was flat enough to fit into a large letter. It wouldn't hurt to call ebay and try to have it removed.

Maybe in future listings, you could include photos of the way it will be shipped so there is no confusion. And to be really sure, a detailed explanation of your packing process. Make it foolproof, so it can't happen again.