Since when is a parcel 'lost' in the post.
It's not been lost it's been STOLEN.
What makes you say that?
Bearer bonds aren't normally sent via PM so there would be a case that you were negligent in not picking a suitable company. Your negligence would have to be considered as well as any by PM.
Do you have any other unrealistic and silly examples?
Here's a fairly realistic one:
Let's say someone sends a GTX 680 via PM, something entirely fathomable and the sort of stuff one would expect to be sent. They insure it for £50, it gets lost. Why should PM have to pay £400 when they accepted the contract on the understanding that if they lost it, they'd only have to pay £50 as that is what you told them it was worth?