I don't see how they can deny that claim. I have dealt with the claims procedure a couple times. One was denied and one was approved. The one was denied because the seller didn't put a cyl head in a box rated for that weight. I don't see that being an issue with a radiator and I'm sure you packaged it well to begin with.
Edit: Just wanted to add I was the buyer on both of these. Nothing I have ever sent out has had a claim put on it or arrived in damaged condition.