Was the letter adressed to you? Was it registered? Never sign for any letter unless you are expecting something important! If they want your sig, it's gotta be bad. Depending on the post office rules there it is an idea to used a different name on any pkg as well. Most POs will allow you to P/U mail if the addy on your ID matches the mailing addy. The deny, deny , deny route is usualy best but in SU I realy don't know. The fact they are dangling a promise of not giving you up to IWC tends to tell me that they are on thin ice here. Why make promises like that if they are on firm ground? If it was cut and dried they would just say "You owe us, pay up." Is there a free law advice service in SU? Ask them for guidence if there is as Swiss law is wierd and tough.
Also, move countries
Good luck and regards, Col. (who thanks his lucky stars he does not live in an EU police state or SU).