Unhappy with defaulted debit irder charges.

Good day, I have a 24 month contract with Vodacom and I defaulted with a 1 month payment. My payment is via debit order and payment of R 106.00 goes out at the end of each month as per the contractual agreement. My problem is I have been calling Vodacom on several occasions to change the debit date to the 25th of every month. I asked them to check my calls for quality purposes. They asked when can they debit again and it was agreed for the 16th and the money was still not available. Now I know due to this, there is a penalty but for it to be so large an amount shocks me. The payment including the current installment should be R 212.00, I had to pay R413.76 a difference of R 201.76. I also had to pay the bank debit order R7.00 insuffient funds fees. The bank is about money I understand, but Vodacom is about cellphones etc. to make money how come their interest is so much more than the banks why are their fees so astronomical. My service was not even cut of ? for the charges to be so high. I am a pensioner and cannot wait for this contract to end.