So if you're making just the minimum payment, you're paying mostly interest. You'll reduce your balance quicker even if you pay just a little more. Some cards, like your Citibank card, give you an interest-free grace period. That means if you pay the full balance by the due date, you incur no finance charges for that month. On the other hand, if you carry that balance to the next billing period, you'll begin paying interest, based on your average daily balance, all the way back to the original purchase date.
Cash advances are different.
When you get a cash advance or use a convenience check, interest is charged from the date you take the cash to the date it is repaid in full. A grace period is NOT an extension of your payment due date. If your payment is received even one day past the due date, it's late. That means late fees and a blemish on your credit rating. It's better to mail your payment early so it gets in before the due date.