It might be cheaper to hit the liquor store and grab a bunch of boxes, use a box cutter to cut them down to fit, stuff the bag, and then mail/FedEx/UPS it. Empty boxes are light, but they should maintain the size and shape of the case as long as they don't toss it on the bottom of the stack.
Maybe add a sampler bottle of liquor from the store and put a "fragile" sticker on the case?