Problem with shopping cart.

This is weird to me but the more I use this theme the problems I find. I have had it for over a year now and I am pretty disappointed with the way the cart works. The latest problem I had is a customer bought 3 of one item, when my quantity said 1. So I am short 2 items. See Attachment. As you can see from the picture I have a quantity of 5. But if type 20 where the yellow arrow is and hit the buy now button it will add 20 to your check out and you can pay for all 20 but I only have 5. Does anyone know how to stop this?