Charged after 1 month from cancellation

I called Telus to cancel my service on July 26 and my service was cancelled on August 1st .

But I received an email today saying that I had to pay 87$ and the bill says that it was applied on August 27 but ; 

WHY , my service has been cancelled for over a month and on the webpage shows that my account is cancelled. 

Why am I getting charged for a service that I did not use? 

Help...