If you already paid, you can safely ignore this. Just to be sure, log out and back in again and see if the bill is still the same, a few have reported a bug where the same bill was mailed twice but when logging in, a new bill showed. Worth a try :)
Andrew, there is no error regarding your account. There is an outstanding amount that's due Feb 28th. You will always be paying one month in advance with all mobile companies as that is the standard approach.