Unnecessary interest charged

I was charged over R400 in interests for missed instalments. These people debit whenever ever they want. The second installment was due in less than 30 days after the first initial instalment. Every fourth day a debit order was sent to my bank account, and R95 was charged after every unsuccessful debit. I have indicated that my debit order must run on the 25th but payflex wanted to benefit R95. I will never recommend or use this facility ever again.