[kansface]

What would the post office do with spam? Their existing business model is chiefly predicated on delivering junk mail.

I’m not sure how the federal government would deal with fraud on the payment side, either. The US does not have a strong system of identity.

[wombatpm]

Junk mail is advertising mail that someone paid to send to you. You what it is not? Illegal. Scams, fraud, and other illegal things get shut down because of postal inspectors. And there is no anonymity. The USPS knows both ends of the transaction.