Not sure how this happened but it has happened twice in the last 24 hours.

I have an item for sale and 2 people have purchased it with no problem. It is a file but I don't have the file there. Instead, I email them the file after I get notice of their purchase. 2 people have purchased it with no problems but today I had two people purchase it and it let them use the Free Charge Card so it didn't charge them anything. I have it priced by attributes and I have tested it by going there and adding it to my shopping cart and it shows the correct total all the way through the checkout process. How in the world were two people able to purchase it and use the Free Charge Card ?