Cell C madness

CellC has now completely made me lose my temper, I have been a customer for a good 20 years. 6 weeks ago when I opted to upgrade, they got my order wrong, not only did they get it wrong, they didn't even log it as an upgrade, but as a new contract. Needless to say that when the devices were delivered, ( 2 for 1 deal), it was the wrong devices, so I sent them back, they then sent the wrong devices again. This was after I had to continuously contact them to find out what is going. So I requested for a cancellation. Well here's the kicker! They did not do the cancellation, and still wanted to debit me for 2 separate contracts which is not what I wanted to start off with. I was then told the cancellation was successful before my debit date, it wasn't. So after I we as advised the cancellation was done I could now apply for my upgrade. Just to find out that no cancellation had taken place and I no longer qualify for an upgrade. So I reversed the Debit order, due to the fact that it was incorrect. I stipulated to them I'll gladly pay my invoice once corrected. So today I receive a mail stating the cancellation has now been done, but this has affected your credit score, so you are no longer eligible for an upgrade. What do I do?