Credit card purchase interest is what a credit card issuer charges when you don't pay off your statement balance in full by the end of the billing cycle in which the purchases were made. The purchase interest charge is based on your credit card's annual percentage rate (APR) and the total balance on the card. With most credit cards, you are only charged interest if you don't pay your bill in full each month. In that case, the credit card company charges interest on your unpaid balance and adds that charge to your balance.