Ridiculous Bank Interest Charges

I had a terribly bad experience with absa,I was retrenched 3 months ago,and for the past 2 months my few debit orders were unpaid,only to find out absa has changed me R938.19 for upaid debit orders of R600.00,when I call and ask them they tell me they charge R151.00 per unpaid debit order,I find it very ridiculous for a bank to charge R151.00 for a R18.00 debit order how much interest is that,is it even legal for them to charge that kind of an interest?I am very disappointed with absa and I want other clients to know about this *******,I am going to close down my account with them .