Just because a package shows as delivered is not proof that the USPS actually delivered it to the right person. My sister had a package from
Amazon go missing this week - it was marked as delivered by the USPS, but there was no sign of it. It is also well known that the USPS will show a package as delivered hours or even a day before it actually is - think about it, if they are doing that, then there has to be cases where they mark it as delivered when it never is.
As a seller, while I guess you can try and play the "well, I have tracking info that shows it as delivered - it's your problem now", that's not good customer service. As a seller - even an occassional one - you should consider yourself a business, and work with your customers accordingly. You should contact the post office and let them know the package was reported as not received by the addressee. Also, as the shipper, you will have responsibility to file the claim against insurance. Take this as an opportunity to show your customer that you value their business and work to help resolve the problem - if you don't, well then, I personally think negative feedback would be valid. (And as I buyer I do read the responses where sellers state "tracking info shows as delivered" as their only response and avoid those sellers - because I think that response shows a lack of customer service.)
Think of it this way - if you ordered a package from Amazon, but never received it, and they just told you "too bad, so sad, the tracking info shows it as delivered", how would you feel about them????