Got charged a late fee, yet paid on time

posting here because I've been unable to get ahold of Xfinity by any other means. I have every receipt and confirmation of payment for the entire time I've had Xfinity, yet today they claim that I haven't paid for February (which is a short month) even though the bill is due tomorrow and I paid today on time as usual. I have a 13 dollar late fee that I don't owe. how do I get in contact with them to resolve this?