Bill sent after account aet as pay as you go

I HAVE CANCELLED MY ACCOUNT AND HAVE ENSHURES THAT ALL AMOUNTS DUE ARE PAID. WGEN CANCELLING THE ACCOUNT, I ASKWD THE PERSON IF ANY OTGER ANOUNTA ARE DUE AND I WAS TOLD THAT NOITING IS OUTSTANDING. I GOT AN MAIL WITH AN AMOUNT DOUBLE OF MY NORMAL BILL I USED TO HAVE. HOW THE HECK IS THAT POSSIBLE IF MY CONTRACT IS SET TO PAY AS U GO???? !!!!!!!!!!!!!