[gpvos]

In fact, in some European countries it was until recently general usage to prefix a postcode with the country abbreviation for cars,[0] i.e., A for Austria, B for Belgium, D for Germany, FIN for Finland, H for Hungary, NL for the Netherlands, SLO for Slovenia, etc. But I guess the ISO country codes are now at least as well recognized and will also work.

[0] http://www.columbia.edu/~fdc/postal/#europe "After World War II and up until the mid-1990s, all European postcodes included country-code prefixes. These were originally United Nations "car codes" (one, two, or three letters), kept in an annex, "Car (Or Road) Distinguishing Signs", to the 1949/68 United Nations Conventions on Road Traffic, adopted in part by the European Conference of Postal and Telecommunications Administrations (CEPT). These codes were not accepted by the Universal Postal Union as a world standard, but were widely used anyway."