The burden of proof lies with your company to show licensed use of the images in question.
I love how they transfer the "burden of proof" to you. Essentially they are saying "hey, it's on our web site and we claim that we are the exclusive representatives of this image." That's BS! The burden of proof is on THEM to prove that they have the rights to represent this image.
Look, I know many people seek closure. They want an end to worrying about this. But in my opinion, sending them $500 is like throwing it in the gutter. (Worse, because you are re-enforcing and financing their racket.)
Also, I stongly suggest you look into what SoylentG mentioned about NCS sending collection letters to non-debt in Canada. Their claim that they are not trying to collect, they are only trying to "assist you in a resolution with Getty" is laughable. Ask them what the "C" in NCS stands for.