Missing painting. Advice needed!

Whatever the truth of the matter is, the card company had custody of the painting and therefore, I would assume, liability for its loss.  Although you probably feel you have to try, I doubt you will have much success with the courier/Post Office as their contract would be with the company who paid for the delivery. Certainly get what help you can from Citizen’s Advice and consider investing in a solicitor’s letter - you ought to be able to recoup the cost as part of the compensation. Sadly, I suspect that you won’t see the painting again, and it may be difficult to establish a value. Given the circumstance I would consider asking for all the profits from the sale of the cards - I doubt the card shop would be helpful in establishing that, though but it seems morally wrong that they should profit from your loss. It does sound the kind of underdog versus business story that your lol cal newspaper may  be interested in taking up. At least that might cause the card shop some embarrassment and bad publicity if nothing else.

My thought was the local newspaper but local press is struggling now and often covers a very wide area. But what about local tv news? Because a young person is involved you may be more successful. Good luck ! I certainly feel incensed about it too.