Hidden Charges from FNB

FNB bank, they had hidden charges. If you apply for the Credit card and pay full amount before debit order the still they deduct minimum payment. When I called them then they told me if we pay before 5 day's then minimum amount wouldn't be deducted. I did asked them if it is written somewhere then they told me, no it is not in the agreement but there system works like that. Not sure what is my mistake in this.