[phobetor00]

I would try to be imaginative with this case, as they seem to be overly defensive with the CCTV bulshit.
Keep your story and ask them if they can verify with the shipping company if there was any replacement in packages or any incidents in the distribution? Tell them as well you took the initiative of contacting the shipping company, and you were told they couldn't verify the senders as the box label might have been damaged along the way.

Let them proceed with the investigation for you, stressing the fact that you're the one at loss and that you're counting on their full support.