In order to avoid interest charges, you would want to use your card for purchases you can afford and pay them back in full by the payment due date each month. (discover.com)
You'll pay a flat monthly amount which covers principle and interest by a fixed due date each month. (sapling.com)
Your federal loan servicer will assign you a specific due date each month, but getting your payment in earlier can work to your advantage. (creditdonkey.com)