I’ve never had anything to do with shipping art, but I do semi-regularly ship scientific lab equipment for remote experiments. It is kind of similar to art in that it is unique, irreplaceable, sensitive to shipping conditions, and usually tax-free. My team has had consistently satisfactory results from Fedex “special services”. They essentially have worked with us to ensure that the special shipping requirements of our equipment are met, and also handled the legal issues if any. I’m sure UPS and DHL have similar capability.
Best of luck,
saunders111