I would advise a trip to the repair shop to complain and tell them you will report them to microsoft if there is any bother over the key. You certainly do not need to buy another key.
Or you could just change the key to the one that should be on a label on the bottom of the laptop and then activate it. Sometimes the label is underneath the battery so you have to take battery out to see it. How to change key click here
To find out the key that is being used at the moment, before(if) you change it, use the free program Magical Jelly Bean Keyfinder click here to retrieve windows product key click here