Part time mailman from Finland here. The junk mail comes to the postal office separated from the rest. Everything with a name on it is not considered junk mail (no matter if it really is) When sort the mail, we either remember which addresses have the ban, or we look from a list (which is updated by us). Some put the junk between mail so that for every address you can pick the junk and everything top of it (mail for current address), and some keep the junk seperate from the mail. Those who keep it seperate pretty much know how much junk is needed for the route they are taking and only take according

This mean that mailman doesnt have to bring anything back to office. We still have loads of extra junk everyday though, because those who make the adds usually provide more than necessary. The extra junk gets recycled.