A cargo shipment was rejected by Southwest Airlines last week in Little Rock because it didn't meet the labelling requirements. I'm not sure why they didn't just return the package to the shipper, or perhaps it was labelled as to the contents, just not according to the rules. It contained a few dozen human heads. I imagine someone opening the package, a bit of reflex shrieking, then a whole lot of people from all over the operation being summoned to come see. They called 'the authorities,' presumably that means the police, and the police brought the coroner. The heads were on their way to a research and educational facility, so here doesn't seem to be any reason to suspect foul play, but they are investigating to make sure the body parts had been legally obtained.
So there's two things I never thought about before. One, the existence of a black market in severed heads, and two the proper labelling procedure for a shipment of severed heads. I don't have any more information on the former, but an acquaintance in the airline business forwarded me information on the latter.
Your severed heads need to be properly packaged in an outer hardshell container, no particleboard or cardboard, with other packing requirements as outlined in the document linked above. Then you must clearly mark “Biological Substance, Category B” in 6-mm-high text on the outer package, adjacent to a properly sized UN 3373 diamond-shaped marking. It does not appear that the container must also bear the legend Contains Human Heads. So to be on the safe side, just assume all biological specimens are severed heads.