R120 for an unpaid item, why are we being ****** like this?

After many years of banking with FNB i had my debits go down and miscalculated what i needed. I have 1 item bounce for R1515 which I will now rectified via eft. My question is how on earth do they charge me R120 for an unpaid item. My living costs are R20k p/m and thus should warrant that this be stopped. I am utterly shocked at the amount and need FNB to please explain how they derive to this absurd cost (do not respond with its market related) . Let me tell you, i am seriously rethinking who im banking with and how in conducting my banking.