Mislead by sales agent

I have never been so upset about bad service before, I was called and took out a laptop, already having an existing cell phone contract. I was told that I will pay my first instalment in January, come December I received a notification stating that i need to pay an ***x amount, which i did not budget for. I am so furious as this is misleading customers into getting the sale. To top it off, when I called customer service to enquire about this notification they inform me that I need to go into the store to have this sorted or If i would like to take this further. I really do not mind paying the ***x amount, what upset me the most is how the agent **** and mislead me to believe that I will be paying for this product only in January. Please do better Cell C