On the checkout page, the field for the ZIP Code or Postal ID automatically changes depending on the card used. The first six digits of a card number (including the initial MII digit) are known as the Issuer Identification Numbers, or IIN's (also called Bank Identification Numbers, or BIN's). These identify the card-issuing institution that issued the card to the cardholder. The rest of the number is allocated by the card issuer.
To sum it up, if the buyer used a card issued in Canada, Postal ID is asked. If a card is issued in the US, the ZIP Code is asked on the checkout page.