My payment was due on a Sunday and I understood that meant I had until the next business day (Monday) to pay. I went online to my card issuers website and made a payment on Monday, but I was charged a late fee. Can they do that?
Yes. If your payment is due on a day on which mail is not delivered (such as a Sunday) and you mail your payment, you cannot be charged a late fee if your payment arrives on the next business day.