Leave any of the balance unpaid, and you owe the interest that's accumulated over the last year, not just interest on your current balance, says Bourke. That often surprises consumers, he says.

Additionally, if you make a payment 60 days late, you forfeit your deferment period and zero-percent financing. The new APR could be a penalty rate that's higher than the regular rate that would have kicked in once the deferment period ended, says Wu.

Another point that confuses some consumers: When is that final payment due? Your billing cycle may not coincide with the end of your deferment period. In that case, be on the safe side by using the end of the deferment period as your due date. That date should be displayed on every bill, says Wu.

One solution: Set your own payoff plan to have it paid in full well before the due date, says Kathleen Day, spokeswoman for the Center for Responsible Lending.

Be sure to ask what payment methods are accepted, says Day.