I here the postal workers asking all the time if someone wants insurance on something. Most of the time they say yes. They have no idea that if the package goes missing, they will have to provide proof of the value of the contents.
*** That is no different than auto insurance, homeowner's contents, etc... I have a 1969 Firebird 400 whose value far exceeds its original cost. You have to show reasonable current value, or the actual sale price of the item. I see this regularly. The post office used to pay up on claims with a very minimal amount of fuss. The explosion of eBay sales, and thereby shipping volume, caused a huge spike in claims, many of which were items not damaged in transit, or were damaged but claims were being made for over the sale price. So the USPS went from being very lenient on claims to being very hardline as to the actual worth. In some instances I think they make it very hard, almost as if they automatically disbelieve every claim turned in.
How many of those people mailing packages actually have receipts for the original purchase price of their items? Your insurance is useless unless you can prove the value of that package.
***This is very true. But a seller, as opposed to someone sending a family heirloom to someone, should have reasonable proof by way of sales receipt/email messages leading to the transaction/bank deposit/certified check/etc.. to build the proof needed. It still may take a little time and you may have to answer questions, but you should get satisfaction eventually. I have seen emails between buyer and seller used as proof. That is actually kind of weak, since they can likely be edited. But I have personally seen them approved as evidence. Family heirlooms and personal items are a different story. It is harder to prove a value to the USPS for them.
I sure hope you get the guns back.