Closing the card immediately after you've paid off the debt could undo some of the good work you've done with the zero-interest offer by reducing your available credit, lowering the average age of your credit accounts and affecting your credit mix. Together, these three factors account for 55 percent of your overall credit score. The easiest strategy is to keep the zero-interest card open and use it occasionally to keep the account active while continuing to pay off the balance on time.