Been dealing with this exact scenario recently. Here's an abbreviated version of a very long story that's still ongoing:
-Listed a watch for sale here and on WUS in October.
-On 10/24 a buyer reached out with an offer, I accepted. He paid via Paypal right away. He asked to hold off on shipping for a week since he's travelling.
-I shipped via USPS Priority Mail 11/2 from Chicago to Texas. Priority comes with up to $50 in insurance, I insured privately for the full value.
-Per tracking the watch was last seen at a hub near me on 11/3. The tracking said "Your package will arrive later than expected, but is still on its way. It is currently in transit to the next facility" at this point and has not been updated since.
-The buyer (reasonably) got antsy, so I refunded him.
-I opened a customer service inquiry online, spoke to the local post office in person twice, and called the post office hub during the first 10 days or so.
-I opened a Missing Mail Search on 11/10 - (this is a formal search and has to be done before filing a USPS claim)
-I opened a claim with the USPS on 11/23 (Priority mail claims had to be opened at least 15 days after scheduled arrival - I gave it a few extra days at their suggestion due to election-related delays).
-I opened an insurance claim with the private insurer (Secursus) on the same day
-The USPS processed and approved my claim on 11/25, I received a check from them on 11/28 for a refund of the shipping expense plus the $50 insurance
-The private insurer hasn't made a decision but from the questions they ask me they are looking for a reason to deny the claim. I'm really frustrated because the USPS very quickly paid insurance and very clearly has lost the package. This has nothing to do with anything I or the buyer may have done outside their T&C....The package was lost in transit. I can't imagine a more straightforward claim.
Hope that helps. I've shipped hundreds of packages domestically using Priority Mail over the years, and this is the first one that was even late, let alone gone missing.
Good luck on yours!