I received a phone call and a subsequent email of the Post employee
which assisted me in filling out the form, saying that the adress is
set up now but that I won't receive a formal confirmation and that the
Fr. 20.- are to be paid at the end of the year.
So, if anybody wants to test, just send a postcard/letter etc. to
"Wikimedia CH, 8008 Zürich" (and please, please, don't spam my post box now...)