Cancelling my AT&T Account - What else do I pay for asides from ETF?

Can any one explain to me the logic behind paying for something you don't use? For example, if someone wants to switch to different carrier at the beginning of their cycle, they have to pay for full month, although they have cancelled? Shouldn't the cancellation be immediate?

According to the terms of service from most cell providers, the final month of service will not be prorated. If you cancel during a month that you already paid for, you will not get a refund for the unused portion.

If I switched carriers and had my phone number switched to my new carrier, how do I lay my last bill? I always used online billing and paid from my account 

You should still receive an email notification of your bill, and if you had it setup, a notification when the bill was "due".

Once you port your number, your old account will be closed, so you will not be able to pay online any longer, but you can still pay by mail, or go into any corporate owned AT&T store to pay your bill.  You should also be able to pay your bill by calling the AT&T toll free number for the billing department.