Sent an item last month via Priority flat rate small box. Did everything properly. Printed the label here, insured for the full amount paid, properly packaged and shipped. They lost it. After jumping through their hoops to file the claim, providing "evidence" I paid for insurance (it is clearly visible right in the tracking details), they sent a check and a letter stating "based on our investigation, we have adjusted payment for the proper amount" and it is roughly 70% of the amount the buyer paid (and clearly shown on multiple receipts of the transaction I had to provide). Of course I appealed it, but I am not feeling too hopeful.
So, anyone out there looking at having to file claims for items lost in this debacle, don't expect a smooth process. Anticipate having to appeal. And save those shipping email notifications until the packages have delivered.
