Transaction authorized and deducted without the secure input of an OTP number ...now refusing to reverse it

I shop online and know that NO transaction will be authorized without you entering the OTP nr fnb sends you when you buy something online. Well I am WRONG! I was looking at the keto pill and the merchant had a promotion online for R410 incl shipping fee. Yes I typed in my bank details but while waiting for the OTP nr...which did not come through at the time, so I cancelled the transaction...thinking it wont go through as I did not type in a OTP number nor did the secure site pop up where I'm suppose to type it in. But FNB authorized the purchase and the amount taken off was R2991?? So I tried to report it as ***** and get the amount reversed but I was told FNB cabnt reverse it. Instead I'm told they will open a dispute on my behalf??? What **** is this? If you can reverse out debit orders you can surely reverse back unauthorised purchases...especially If I made effort in letting you know it was an error and again I did not insert any OTP numbers what so eva. So again I ask.....HOW CAN FNB AUTHORISE THIS PURCHASE WITHOUT ME PUTTING IN THE OTP NUMBER????