Charging after account was cancelled

I have had a very difficult time with this ISP since the onset. They are constantly not available to address your issues. Eventually decided to change my ISP and go direct to Telkom. Cancellation of my account and Capped package was confirmed and I was set to move. I still had 3 domains with them which I am willing to pay, but they also hit me with an uncapped package that they claimed had to be cancel. If I was always on a capped package and that was cancelled and confirmed, then why would I need an uncapped package. Support and accounts are not willing to waive this erroneous payment and I am not going to pay it either. If it comes to litigation I will defend and claim damages.