Why was the royalty payable only on unused stamps? Surely if a royalty was due, it should be on all stamps that used the image?
The stamps that were used were used as payment for mail transport and delivery. There was presumably no profit there (it's not a company trying to make money), so no net benefit to the USPS from the use of the photo.
I doubt an argument based on lack of profit would hold in other cases of intellectual property infringement by a major corporation.