Most NPSL cards are charge cards, requiring that you pay your balance in full every month, which is why they have no interest. Interest might apply to past-due balances in the form of late fees. For instance, the American Express Plum Card charges a late fee of $39 or 1.5% of the past due amount, whichever is greater, if you miss your first due date. If you miss making your payment for two consecutive billing periods, the late fee increases to $39 or 2.99% of the past due amount.