Fair enough. Most advertising isn't individually posted to each address because that's expensive - they hire their own guy, who isn't a postal worker, to go around and put it in everyone's mailbox. It's like paying one cent per email to prove it isn't spam.
In the US it's illegal for someone who's not a postal worker to put things in your mailbox for exactly this kind of reason.
No, 99.9% of the mail I get is trash and is delivered by USPS. You can't even recycle most of it because of the paper they are printed on. It's a huge, disgusting waste of resources on multiple levels.
At least in the states, the postman is almost assuredly doing the end delivery of the mail, and is often given a stack of advert materials to mix into the delivery for a certain delivery area. The use of a mailbox by anyone who isn't a USPS employee delivering paid mail is prohibited by law. https://about.usps.com/news/state-releases/tx/2010/tx_2010_0...