MTN overcharging

I recently did a upgrade at this specific store late October, my tariff plan on the policy documents signed said it's R349 x 24 months, when I asked the staff at the store if this is correct they said yes my tariffs would be R349 I than did my second upgrade and it was at the same tariff I feel this is ********* business as what I signed for is not what I am being billed I spoke to call centre staff 2weeks ago no response I went through to the store and they told me it's not what's on my documents it's what the system says! I feel cheated and this is ********* business practices as I would not have done the second upgrade. Nor went ahead with the first one.