UPS stores are franchises (Mom & Pop stores) and charge what they feel they can get away with. They will only insure your package if they box it.
Go to the Mesa Hollywood store and ask for a spare shipping carton. They get them all the time, don't be embarrassed.
A Mesa Boogie box for a short (18") head measures 33" x 15" x 15".
Any box that barely fits the head will be way too small and provide little protection.
You will notice that Mesa uses 4-5 inches of styrofoam packing for the corners. There will be 3-4 inches of space around the entire head inside the box.
If you use UPS, they will charge the buyer a nice, fat brokerage fee on top of whatever else Switzerland wants. Same with FedEx.
If the buyer refuses to pay, they will charge this fee to YOU.
I know a lot of people distrust the Post Office, but I have used them extensively for overseas shipping without incident. Go Express if possible (It is expensive, but it's also only three days in the system), and pack well enough to withstand a four-foot fall from the back of a truck. Believe it.