Incorrect invoice

I went to MTN Nelspruit 30 April 2015 to migrate my package to MTN Sky - my mobile number is *** I was told that it has been done and will take 24 hours, I decided to follow up with MTN call centre on the 11th of May and spoke to Xolani Hlongwane and he informed me that I was still on my existing package and that they have NO record of a migration to MTN Sky. He sent me forms to migrate to MTN Sky which I completed and the migration took place on the 12th of May. I then received my monthly invoice for the billing period 11th of April to 11th of May which is a bill of R3654.59 which is a billing for my existing package which is correct but also for MTN Sky of R1402.63 which is incorrect as I have not been on the MTN Sky package for that period as MTN Sky only started 12th of May. I emailed Xolani Hlongwane and said please correct this mistake and send me a response his response \You were charged for the new MTN sky package on this invoice plus what you have used before the migration was done "which is not acceptable and I demand now that they take the MTN Sky subscription of for this period and correctly bill it on the next invoice which will be 12th of May - 11th of June"""