UNAUTHORIZED DEBIT

Purchased goods last week just over R400 via the online store , the money was paid from my bank account , and last night another transaction for R180 was debited from my bank account from Woolworths which was unauthorized call the customer support and spoke to Nishaad and she advised this is normal when paying with a cheque card (she was very helpful) but my problem is why did this debit occur i purchase often from the online store and this is the 1st time this occurred, kindly check and advise.