Dstv stole my money

I upgraded my DSTV Pricelock package to a new one because my old one has expired. hire is my story Dstv contacted me the end of February 2018 telling me my pricelock is going to expire on the 9th of March. I told them I am interested in upgrading to a new pricelock plan. Dstv told me if I receive my new decoder I must switch off my old one so that I would not be paying for two contracts. On the 3rd I received the new one and Dstv told me I cant switch off my old one because my old contract will expire on the 9th. So I waited till the 9th and I got a installer on the 12th to make the change to the new package. I personally called Dstv after the installation to make sure there is only one subscribtion active, and the adviser on the other side told me everyting is fine I don't need to worry a thing because theres only one subscribsion active. But now on the 3rd of April I got a bill of R2539-81????? I called them and asked what is going on. They told me I didn't switch off my old subscribsion???? I told them to ge back on the telephone recordings and they will see that it has been cancelled. They rep****: We will come back to you. They did come back with a offer of R765. can somebody give me advise how to get out of this and get my money back? Thank you