What is the address for returning KPN box for internet?

Hi, I am at the post office trying to return the kpn box I initially received when connecting to the internet and now need to return it, but the post man is asking about an address where I should return it and I have none. Where should I finally send it? I was informed în a kpn shop that I am able to return it for free at the post office but no one mentioned the address.