I'm having the same difficulty too, can't make any purchases in iTunes etc as it won't accept my postcode, it keeps coming up "Postcode Invalid".. I've also tried the 6 zeros but not working. I'm waiting for Apple Support to get back to me which was meant to be today (48hrs later). Any advic? Thanks ?
Your postcode is linked to your billing address, so you get it from your address. If you are being prompted to enter it and if you don't think that you have postcodes where you live contact iTunes Support :