On USPS in DC - I've had packages marked delivered which have not been (usually, they turn up the next day), and have seen tracking updates telling me I've signed for a package on days when I haven't been home. There are good guys, and not so good guys amongst their staff here, but one thing I've noticed here is that they do tend to rotate the routes fairly often, so you don't have the same person delivering your mail for more than 6 months or maybe a year. So often you don't know your postman that well.
Lost parcels are still pretty rare in the big scheme of things, and if you send a lot of parcels, I'm not sure the premiums you pay for USPS insurance make sense, especially as they will try their best to wriggle out of paying for it. One of the parcels that I had go missing was a box of knives I'd sent to a popular kydex bender for sheathing. When he sent the box of sheathed knives back to me, it was left on the porch (despite notices saying not to do so), and stolen. USPS did pay out for the insurance claim, but because I didn't have receipts for the knives (I'd had most for some time), they only paid for the value of the sheaths, not the knives. To me, this is a complete con - if you pay to insure a parcel for a set amount, and it is lost, you should receive the amount of insurance that you paid for. Having to prove the value of the package makes no sense - if they're not willing to accept the value of the package, they shouldn't sell you that amount of insurance in the first place. Of course, if you were dealing with damage or partial loss, then it would be different, just in the same way you need to evidence the value of what you've lost under home contents insurance.
I'm sure USPS will make Fullflat jump through hoops for the claim (that is how insurance schemes make their money, after all), but I'm optimistic they will eventually pay it (and happy to do anything I can to assist the process). That still won't make up for the fact that a really unique knife is lost, and will probably end up with someone who has no appreciation of its true worth.