Try going to the giftcard website of the giftcard and look for a way to register the card to your name and address first. Once it is identified with your name you can generally use it for internet purchases. Occasionally there is a mix up when they add the extra (4) zipcode digits to the end automatically. Ask them not to do that.
Make sure you do not use the "paypal account" option or you will be charged $1 dollar, just go straight to the credit card purchase option. $1 USD will be withheld as a temporary hold on the card so purchase $1 less CP. (example: if you have a $50 giftcard only purchase $49 worth of CP or it will be denied)
Thank you Gideon! I was able to find the website on the back of the card and register it. Unfortunately, I am being charged $1.25 CND every time I do try to get it to through. I'll give it some time to register and try again later tonight.