Postfinance even sent me a letter because I didn't answer the phone and wanted to offer me services, even though there is a personal mailing box on the e-banking and they have my email.
I mailed them back and threatened that I will just throw anything who has no address or anything like "free samples" back into the big yellow post boxes. And I will!
You have to differentiate between what the postman is delivering and what the local pizza place / beauty salon / window cleaner is posting en masse into the area's mailboxes.
If the mail is advertising but is directly addressed to you by name the post office is duty bound to deliver it to you. They are not duty bound to push non-specific junk mail into your mailbox.
The local school kid who has been offered CHF150 to pepper the area with leaflets from the local pizza delivery shop will NOT give a stuff about your "Keine Werbung" sticker.
I had the same letter, and actually I think it's a nice offer, enough people have a "keine werbung" sticker, but still would like some specific things.
You just get a greater choice, and if you don't respond nothing changes, how can that be a bad thing?
Only unwanted things I get are from car dealers offering up to buy every car, pawnshops who are willing to pay to little for our Jewelry and every restaurant in a mile's range, but those are not delivered by the mailman.