The shippers hate round tubes. They fall of conveyors, and get caught in machinery. I was sitting on an airplane once, and watched my PVC tube with my longbow in it, fall from the door of the airplane and crash onto the tarmack below.
I ship hundreds of longbows a year, and always use long square cardboard boxes and never have a problem.
When I ship something heavy like a muzzleloader barrel, I put it in a pvc or heavy cardboard tube, and put the tube in a cardboard box.