Optima credit card customers are bringing a class-action lawsuit against American Express, claiming that the company engages in unfair and misleading billing practices. The plaintiffs allege that when customers return merchandise charged on their Optima card, the company deducts the purchase from the balance charged at a lower interest rate instead of the balance at the higher rate, and thereby charge the customers more due to the difference in the rates. Card members pay 3.9 percent interest for balance transfers from other credit cards and 14.99 percent for regular purchases. The suit also claims that American Express unfairly inflates the cardholders' average daily balance by accruing interest each day on outstanding debt balances. Plaintiffs say that the monthly interest charges are increased by these allegedly inflated daily balances and that both figures lead to higher finance charges.