An insurer only needs to pay 1) for a loss that was insured and 2) which is provable. If you had custom add-ons to your truck which would increase it's value beyond that of a stock truck and wanted them covered, they should ideally have been disclosed when you bought the insurance, and would possibly have affected your premiums--i.e. expensive options or accessories often need to be specifically insurered. If the insurer is willing to look past that issue, as they seemingly are, they still have a right to demand some proof of the installation and cost of the items before paying out for them. The insurer does not need to take an insured's word, without proof, for the fact that there were expensive add ons.