Ok. Would you do only that or also one of these options

* put an error message "you are trying to use a card issued in X from Y, contact support"

* put a prefilled unchangeable field containing the current country

* signal an error on all fields when I insert something invalid (say, country and city) rather than only the zip code

* block the transaction before you validate my card

Because if so, you're better than what is currently available. If not, I'd be happy to understand what would be the downsides of doing these things :)

EDIT: also I realized now: you'd flag me if I put a non matching _billing address_ rather than if I use a card from abroad? That seems counterintuitive :)