I agree with @Puntin. The Postcard ID goes after where you live and where your mailbox stands. I live on the border between Germany and Austria. My postal address is in Germany, so my postcard ID is DE. However, when I try to send my postcards, I often can't find partners as there are many very active members in Germany. When I then select Austria and Tyrol from the drop-down menus, I can send my next postcard by dropping it off across the border. Unfortunately, some members accuse me of violating the rules and will grade my postcards lower.