Welcome to the Xfinity Forums. I can imagine that would be a surprise! No, you should not be billed an entire month's rate for a late fee. If this were to happen, it sounds like there may be something else going on with the bill. I'd be happy to review your account to help get this figured out. Can you please verify your first and last name in a private message? To send a private message click on my name "ComcastChe", then click send a message.
Also, I wanted to let you know "What to Do If You've Received a Late Payment Notification". You can also "Set Up Xfinity Text Alerts - My Account Help". If you change your mind, you can "Stop Receiving Comcast Alerts via Text or Email", or just Text to Get Account Updates with Comcast Alerts. I hope this helps.