I should not be liable for Cell C's error!

On the 24th of December 2014 I purchased a headset from the Cell C at Westgate. The advertised price for the contract was R429 for 5gig anytime data, 5gig night time data and 25mins, on an open line. I asked the sales lady if I could have that contract but with the option to top up should I reach the maximum instead of it being an open line. She misunderstood and gave me a 100 top up plan, which I only realised when I got home. I went back on the 27th since the 25-26th was a public holiday and spoke to the manager Tiisetso, who told me he could not do anything since the lady who helped me was on leave. she returned the next day and called me to tell me that I would have to wait for my first payment to be made before we could fix this mix up. I went in yesterday to have this fixed and I was told that I had to come in 24hrs after the contract was activated (which happened to be public holidays). If I want to switch now to the package I want, I will be paying an additional R30 per month for an error that was not my doing. This to say the least is unacceptable! I did everything they asked and came in as soon as possible and managers are less willing to help than the general staff!!