Post office in different countries have different policies. Mine does require the original invoice of the item for the insurance claim, and doesnt insure jewellery.
As for the disclaimer part, its the sellers' way to protect themselves, buyers could be potential scammers too if they received it and claim the item is broken, or item in it stolen, when not.
There's why we often say to "buy the seller before the watch", TD would be the best alternative otherwise.
Either way, I hope something can be worked out between u and the seller here, since it looks like no one is at fault here except the postal service.
Good Luck!