TELKOM service

I have Data contract with Telkom with router - *** - I have been calling every month to change my Deduction date to last day of the month - This seem to be falling on deaf ears - Every month they go into my account to deduct knowing very well that I wont have the funds so they can Charge me penalty of R500 for a contract of R150.00 I have with them .It been going on for a while now and I feel they are doing on purpose just to milk me as much as possible for these R500.0. penalties because I am scared they will blacklist me if I cancelled their contract .Imagine If took RAIN contract for R600 Unlimitted Data - instead of 40 GIG data I am paying with TELKOM R800.00 for because their system can not change the Deduction date so - I get penalised every month ..Can someone help me to get this right or help me out of this contract as i see myself being sucked dry by TELKOM for the next 16 months - for a contract that is supposed to cost me R300 .00 instead i pay R800 monthly because of penalties which are not even my fault ..I called the call centre the ladies are very much apologetic but you can tell they feel so helpless since they dont even know how to help ..My only evidence I have is that ..These call are recorded - if TELKOM can check they will see that I have been calling ever since about this problem .PLEASE HELP SOUTH AFRICA