Junk mail is a net positive for postal service revenue. Even if you could eliminate it (major free speech concern), would it be a good idea? Who would pay extra when the can just throw out the junk in less than 2 minutes a week?
its not just paper - all kinds of strange plastics too
when i was in the states i despised that i was required to accept said junk mail under penalty of law and i think thats also a free speech concern. i have the right to not be part of an audience