Payment discrepancy complaint, account debited without authorisation

I am a regular Sixty60 shopper. Of recent, I have not been satisfied with Checkers Sixty60 actions. I recently purchased some grocery items from them, they were all paid and delivered. However 2 days later i receive a notification from the bank that Checkers Sixty60 has debited my account. Out of concern and panic I called my bank to inquire what was going on. I was told that it was a debit from Checkers Sixty60 however not showing on my bank statement. i then was referred to the bank's ***** department. I decided to call Checkers Sixty60's office to find out what the debit was for, to my surprise i was told by their consultants that it was from a balance I was owing from my last transaction two days ago. I then asked them how come I was not told that I have a balance outstanding even on the day of the order? Where did they get the authorisation to debit my acoount? No explanation given but Instead I was referred to the terms and conditions that I agreed to when i registered on the app, I was told that those terms and conditions are on their website and not on the app however by using the app I'm giving the authorisation into my account. This cannot be legally correct. I do not see on the conditions where I authorised for them to trace my account and take anytime they see fit.