Mediocre service

I had a MX3 package, all was good till an agent called me offering the MXV which I accepted, but this was the biggest mistake ever. I was overcharged, I cancelled the product till today I'm still receiving invoices even though I received a cancellation confirmation for the cancellation department. When they call I have to repeat the same story over and over again, and the outcome of the conversation is always the same "we'll escalate the matter", but come month end I still receive an invoice. Now they have sent me a final demand notice for default payments.