Dear Wanda,
Thank you for writing to iTunes Store Customer Support. My name is Drea and I will be your iTunes Store advisor for today. I understand that you accidentally purchased "OS X Mountain Lion" and you're qualified for a free update. I know how eager are you to have this situation settled. No worries, let me help you with this matter.
After reviewing the circumstances of your case, we determined that issuing you a refund for your unintentional purchase of "OS X Mountain Lion" is an appropriate exception to the Mac App Store Terms and Conditions, which state that all sales are final. In five to seven business days, a credit of 22.99 CAD should be posted to the credit card that appears on the receipt for that purchase.
The iTunes Store provides a warning message that asks if you are sure that you want to buy an item. This warning can be turned off. If you would like to make sure that this warning is on, you can reset the warnings in the iTunes Store by following the instructions in this article: