Cancellation problems

I resontly contacted vodacom to prematurely cancel my contract with them at the end if July. They send me a settelment letter with instructions to sign and send back the letter within 14 days . i paid the settelment next day and send in the letter about a week later. All before the end of July.

Vodacom has now deducted another payment and when i phoned them to find out why they told me that the account was only closed on 11 August and a am liable for the 11 days in august. This is not acceptable . Why should the client pay extra because they take there time to cancel the account?