You can thank the DOT for that. Road flares are HazMat. Meaning they are are PITA to ship. IIRC, the shorter ones have less "stuff" that burns, so you can ship a small quantity of those without paying the HazMat fees. Ship the big ones (and/or larger quantities of the small ones) and you have to have a placarded truck and HazMat certified driver. It's been a while since I was on the trucking company side of the aisle and I don't have any of my Hazmat books any more, so IANAL, YMMV, IACHCST, etc.