FNB gone rogue

I had some items shipped from America, the merchant handling the shipping put through Chargers on my account and at the time no funds were in my account, the first three items charged were purchase declined and I was charged R8. 50 each which is standard practice, however the following ten items they put through, instead of fnb purchase declining those just as they did with the prior one they decided to honour all ten items thus in turn charging me a fee of R120-00 each, which totaled R1200-00 which they refunded only half which I'm still unhappy about as they should have purchased declined all, firstly I don't have an overdraft and secondly I have never given fnb authority to do this on my behalf, their reasoning was that I'm a good client who's account is always in good stead and that's why they honoured said items, so that's how they treat good clients? By charging them huge fees without any authorization from the client. If they purchased declined the ten charges and charged me R8.50 to a total of R85-00 I'd take that no problem however what they doing is ridiculous, as if they have power of attorney on my account. As if today they have solely put the blame on the merchant. How is it the merchants fault that fnb honoured those without my authorization? Is this how fnb got rich? Ripping clients off.