Yes, it was the sellers fault...tough play as it does show delivered.

Back when I shipped with Airborne, I once shipped and it was delivered. Shows as delivered but buyer advised he never signed and got the package. Ending up losing $300'ish. Long and short of it was that by default, which I did not know at the time, by default, they will leave at the home without signature unless you specificed signature required. Whereas with FedEx or UPS, that is the complete opposite and you need to specify if no signature is required....

Can't advise either way as I can see both sides of the story....
USPS...just sucks in general but their flat rate packages are a good deal IMO