I know eBay has "eBay envelope" for like certain collectibles but that's limited by category (that service also has never worked for me), so perhaps this seller just has a custom shipping rule that is that low? But they would lose money on a sale like this. I suppose the hope could be that people buy multiple items at once if they have plenty of (in this case) retro games

Edited - added screen shot of low end rate costs for ground advantage and media mail