You might be just as disappointed as I was when you tried to use your iTunes Gift card to buy an app for the first time. I received a $25 Gift Card as a present for my birthday and quickly discovered that my available credit was not being affected by my recent purchases. I waited a week or so and noticed the charges on my credit card statement. I email Apple support to find out the reason behind this and they replied by only telling me that I could not purchase software, games, or applications with store credit or an iTunes Gift Card in Canada.