I've never done this (successfully), so take my advice with a big grain of salt.

We had something shipped from Canada and it arrived broken. We were told we had to bring the box in and show the way it was all packed. The post office decided it hadn't been packed properly so they didn't have to pay.

In terms of proving value-- I would print out completed auction listings that show what it is selling for. Then spend some time on google and see if you can find some collector's sites or articles that talk about the value of pre-mattel AG items. I believe there is also a blue book for dolls that lists values-- maybe it covers accessories and furniture as well?

Good luck!