FNB don't help when you really need them!

I purchased from an online, US based company (PDFSmart) , a 7-day use PDF converter tool at $1 (so around R18.50) as I needed it to convert a document jpeg. I had no money in my Cheque account so I transfered R50 from my credit card and loaded my Cheque account details to purchase the 7-day use at $1, as I was weary of giving an online business access to my credit card where I have a very substantial credit limit. However, the ********* PDFSmart billed me for a 1-month use after the expiry of 7 days @ $34 which I had no need for, or explicitly authorised to purchase. Despite this, and despite that I had no funds in my Cheque card FNB honoured the ********* transaction of $34 (R636.49) and further, despite that I escalated the ********* transaction to FNB's Card ***** Dispute Debit division, asking that they reverse the transaction, they have helped me in no way. A generic reply stating that if PDFSmart bill me in the future they will help, as I have now cancelled the subscription with PDFSmart. I am so disappointed and frustrated in these interactions with FNB. How could they allow a transaction to place my account in a debit, that I had no anticipated and made provision for, clearly?