How can I establish the value of an item for purposes of a USPS insurance claim? Have you ever done this?

The item is a piece of china shipped to me by my son, who gathered it from my mother’s estate.

It was one of numerous packages shipped to me the same way, with plenty of good packing. This is the only item that broke.

The package was insured for $300 and contained two items. The other one is intact, but the two are a set—sugar bowl and creamer.

As it happens, I bought these as gifts for my mother—I remember the year: 1976—but I don’t know how much I paid or what they’d be worth now.

What will it take to justify my claim, and is there any likelihood that it will ever be paid? If not, why bother with insurance?