I've had two bad experiences with "impact damage". First was 10 years ago when I sent my camera to El Segundo for a "free" cleaning. Dust on the outside does not impact the camera. At that time there were many others of us that were asked to pay so I think it was a management scam. I had to resend the camera to Melville because they messed up all the adjustments in California. It came back from New York better than new (no charge).
Second is when I sent my 70-200 in for adjustment. There was so much impact from shipping that the filter and end element were broken. When I put in a claim against the insurance I paid extra for, USPS said the camera was sent for repair therefore it must have been broken before hand and refused payment. I would never send anybody broken glass (even if I hated them).