I don't care if they showed them holding together the Space-X rocket with the stuff, a little forethought would help you realize there's a reason for laws requiring proper securing of loads, and why roof rack systems are built the way they are, securing your load at multiple points on the car.

Pay for the guy's damages, and count it as an expensive lesson learned, but one that thankfully didn't kill someone or get you arrested. Meanwhile, there is 0% chance that Amazon is going to fork over money for this.