[kevincox]

But clearly cost isn't enough. People are getting spam that they don't want.

You could raise the cost, but that only reduces the problem and inflicts cost on legitimate mail sending. It is clear to me that society does need legislation on physical mail. So your correlation doesn't make sense.

[flowerbreeze]

I'm guessing the cost isn't enough, because it works. Maybe tax commercial letter stamps by 50%, but keep private letters as is?