invalid postcode
what should I put as a postcode (for adding a new credit card) if there isn't a postcode for my area?
If you don't think you have postcodes where you live contact iTunes Support : http://www.apple.com/support/itunes/ww (click on your country's flag, the country where you live, and which is therefore the country on the account ; click on Contact Support towards the top right ; click on Get Help under 'Talk To Us’)