unreasonable billing

I have a cellphone contract which i upgraded end of August 2015 it was a smart plan 50 it did not meet my needs i then went back to the store in November late after i have paid for Nov i asked for change of plan to Smart 100 i now received a bill saying im owing R2500 and something of which R2200 its a penalty fee my problem is how can u penalise someone for taking a higher option which is bringing money to the company i can understand if i was cancelling the contract. This is what i call a **************** im am very angry.