That seems very overpriced to me.
It's mainly compensation for the time spent stocking such inexpensive and common stamps, and for retrieving it from stock if desired. If you were looking for this specific stamp to fill a hole in your US album, 25 eurocents really isn't much to pay for it. You have to pay something
for it, at least a small amount, or nobody will stock them.
Collectors of used US stamps are happy to find a dealer (online or otherwise) that stocks common used US commemoratives individually, or who can fill want-lists containing such material. I know people who resort to companies like Mystic for purchasing individual common US stamps, and they are generally much more expensive.