Passpost card is valid when entering the United States from Canada, Mexico, the Caribbean and Bermuda at land border crossings or sea ports-of-entry. Not valid for international travel by air.
This is important in that you could drive into Mexico, for example, but need to fly out for an emergency, and your card won't cut it. I have both book and card, but I see the card as a domestic travel backup to my driver's license.
