Does the questioner or his cousin bear the sales tax on imported cars, knowing that the cars belong to his cousin and not to him, and neither of them was aware of the existence of this tax at the time of contracting?

If you are merely an agent for your cousin in dealing with the car supplier, then you have no relation to the tax. Rather, it is incumbent upon the buyer (your cousin) or the seller, depending on the agreement or import law. You are merely an agent or intermediary in completing transactions and dealing with the supplier. Your fee must be a known amount or a known percentage of a known amount, and it is not permissible for the percentage to be unknown.