Kathleen, If USPS marks an item delivered that has insurance but the buyer claims it's not USPS will not accept a claim for a lost item because it shows delivered. To USPS that package is not lost, it's been delivered.
I supposed now since most carriers have GSP that shows were a mail carrier left a package you might be able to get them to see if they left the package at the wrong house and either retrieve it or let you file a claim but I don't know if even then they would.
USPS doesn't cover theft, it's up to a buyer to have a secure place for their mail/packages.
If you want insurance to cover items that show delivered but the buyer claims they aren't you have to insure the package like Bygone mentioned above.