Gas filler and modern pumps

I purchased one of these last night for my wagon from Pollution Control Auto Parts. Today my wife got a call from American Express asking about a purchase for $700. I guess my card number got hijacked after I used to buy that part. I wonder how this happens? Was their site not secure enough? American Express took care of the charge so I'm ok. I normally use this card for my Costco gas purchases and Costco instore purchases. Jayson