USPS insurance is a huge scam, and the people running it are likely scum. Based on my experience with two claims involving a broken rifle stock and a destroyed framed print, the only way to have them honor their insurance is to get your congressman and senator involved. Literally CC them every correspondence you have with USPS and write and additional letter voicing your dissatisfaction and expectations. Their policy is to deny everything, regardless of evidence. Know that they're going to do that, and just ride it out. They'll always find some reason for denial, but keep fighting them. On your third and final appeal, if you've involved the politicians and played their little game, you'll finally get your money.
Having been through their idiot process a couple times now and knowing what scum they are, I now photograph everything of importance that I send out. I photograph the item as well as the stages of packaging to show that it was properly done. The packaging issue and documentation of value are the most common reasons they use to deny.
