Unresolved query of "Final letter of demand"

Myself and my mom cancelled her contract that was under my late dad's name in January 2018, in order to go onto pay as you go. There was no outstanding amount. An amount of R116 was asked of us to cancel the contract, which we paid 21 JANUARY 2018. We took the deposit slip to the Cavendish Vodacom shop, which they scanned in and sent to the accounts department (as it was said this needed to be done before the contract could officially be cancelled, which it then was).

Then last month my mother received a 'Final Letter of Demand' (dated July 6, 2108) from Vodacom for an "outstanding" amount of R361.16, for what I don't know.

Yesterday, I went to the same branch to enquire about this outstanding R361.16 and the assistant phoned the accounts department to ask them about it. After a very long process they said it is not something can check on or help me with, as it is now in the legal department's hands. So I ask you who is supposed to help me this and where does this outstanding amount come from?

My mother is on pay-as-you-go and has cancelled her contract and paid the amount due. She is 84 years old and has never received "A Letter of Demand in her Life". This is extremely stressful for her and I am at a loss of what to do.