Bad service

I requested settlements from vodacom for my other 2 contracts,and upon receipt I settled .After settlement, I forwarded proof of payment and cancelation letters for both ro vodacom. Today they send me an invoice indicating monthly installment of the same contracts I cancelled.

Can I really cancel subscription balance and continue with the contract,this does not make sense because I paid money for airtime/data that I was suppose to use until contract end date before even the contract could end.

Now who would under normal circumstatnces pay money upfront of services they never used only to upgrade again,I did not agree to upgrade and now vodacom cannot even provide me with proof thereof ?

I am not happy at all.