Pro rata charges

End of September 2015 , I went to the store to upgrade my contract from 7G day 7G night to 20 /20. while signing the new contract I was not informed about the pro rata that will be charged and there is no where in the contract where it states that when one upgrades they are liable for that pro rate charge. To my surprise when I receive my statement for November , It reflected R798 instead of R399. I went to the shop on Monday and was told to call the customer service. I called and still not happy with the response , I refuse to take responsibility as I was not informed and it is not my fault that their consultants do not know how to do their job.