Hopefully this will shed some light on how the final billing works.
When you cancel by giving the 30 days notice period your are billed up until the date of disconnection. In this time if another bill has been produced before your disconnection date it will show that your are going to be charged for the month going forward.
Once your disconnection goes through any advance charges on your account will be refunded back to you and you will receive a final revised bill with the correct amount owed.
If you are left with any credits on your account after the disconnection of your services you will be sent a cheque refund 45 days after disconnection
The Service you do for others is the rent you pay for your room here on Earth - Muhammad Ali