incorrect debit order date

I went to take up a cellphone contract on the 16 June 2017 at the Telkom Shop at the Bluff Checkers Centre, application was approved, i had my ID,payslip and bank statement. The agent told me that they do not have 20th as debit order date so which date i will prefer, i told her if there is 22 or 25 on the system its fine. July month end the debit order goes off on the 31 july. and at that time i dont have suffient money in the acc, but i made a manuel payment via eft on the beginning of August. September statements there is R200 extra charged of payments bouncing for August. So i had to phone telkom to change my debit order date to the 25th of every month. But i dont understand why was i charged the R200 when the agent incorrectly captured my debit order date wrong and she had my payslip and bank statement to confirm salary date.