It depends on which feature of the card you are using. By default, Delta SkyMiles® American Express cards (Gold, Platinum, and Reserve) are credit cards, meaning you have the option to carry a balance from month to month as long as you pay the "Minimum Amount Due." However, if you carry a balance, you will incur interest charges based on your APR. Most of these cards now also include the "Pay Over Time" feature, which allows you to pay for eligible purchases over a certain amount in monthly installments with a fixed fee rather than standard interest. While you should pay it in full every month to avoid interest and maximize the value of the miles you earn, you are not legally required to do so like you would be with a traditional "Green" or "Gold" charge card that has no preset spending limit.