The green sign reads “Please don’t leave empty boxes here. Bring them to the bin outside”

No it doesn’t. It has a bad slogan in big font: “If we all unpack here, we can all pack up” (the idiom probably works if you squint hard enough). Then “Please don’t leave waste paper (carton) in the self-service-zone. Carry your empty package to the container. The environment will be just as happy as us”.

It does not say where the container is (despite the definitive article in English, it’s not implied that there even is one at location), nor does it have relevant information in large letters, just lots of fluff. It’s accessively passive-aggressive, trying to give the impression that it’s all polite while simultaneously ordering you around.

Here’s what would work: Big: “Waste paper container is around the corner”. Small: “Please. Thanks”. You don’t have to convince people, you just have to make it convenient and they’ll be happy to carry their stuff five metres instead of playing carton Jenga.

Also they’re using “Packerl” for package that’s probably Austria. Maybe Switzerland it’s not like I’m a specialist in southern gibberish. Also not enough yellow for a Deutsche Post shop.