Account Cancellation process and lack of clarity

I had a Vodacom monthly contract which I requested for cancellation on August 10th 2017. The subscriptions centre sent me the settlement letter confirming the amount to be settled for cancellation. I made the payment and received cancellation confirmation on August 11th 2017, and my contract was converted to Prepaid.

For some reason I am still getting notification of an amount for payment. I am not sure why I get this since I settled on an amount given to me. The settlement amount should have included any previous invoice amount that was due and this was not explained to me or clarified to me.

A settlement amount should work on the same basis as loan amounts are through a bank where previous amounts outstanding are included.

Based on this settlement I should not be owing anything further to Vodacom.