phone returned to me warranty claim

Hello. I recently sent an LG G3 back to At&t on a warranty claim. It was sent back to me with a note indicating it was damaged and I'm now responsible for 500$ in charges. This phone was not damaged when I sent it, but of course, I can't prove that. This phone also has insurance on it. I'm an add-on to someone else's account who hhas been with at&t for ages. I don't believe I should be responsible for 500$ when the phone wasn't damaged. In searching these forums I see a whole lot of scenarios like mine.
What do I do? If at&t sticks to this 500$ damage thing I don't have 500$ extra.