Have you looked at your mail lately? The USPS is mostly shipping around recycling, like ads. I would happily pay $7 to send a letter 1 time a year if it meant I would see half as much spam.
Surely you see how the spam is subsidizing letters, and then “public good” isn’t so obviously black and white. I mean we could ban spam, tax to pay $6.50 of every $7 letter to enable wedding invitations be $0.50 to mail… but why?