I would think that the courier, acting as an agent selling insurance accepts the insurance liability if they 1.know what they are insuring (whether it is on a list of prohibited items or not) and 2. They accept the premium. In my simple way it’s like saying to a company I need travel insurance to go to Vietnam, and the company say fine that’s £60, then my case goes missing, they can’t then say that Vietnam is excluded, if I had told them I was going there and they accepted a premium to insure me to go there. Whether the insurance company pays out or not is between the courier and the insurer.