IMPORTANT!
If you're buying gift cards from places like Walmart or Home Depot be sure to ask if they are useable in Canada. In most cases the gift cards purchased on one side of the border ARE NOT useable on the other side for whatever reason. I know this has been the case in the past but may not be now. Best to ask and know FOR SURE.
I'll see if I can get an answer from the Walmarts and Home Depots and such up here but it's always good to double check.
You'd think they would be useable but it's usually because American stores operating in Canada are quite separate from their American counterparts. I know this is the case with Best Buy. They're accountable to the same shareholders (ie: making the same people rich) but both sides of the border have independent Presidents and are run completely separately (and even have different business models).
Cheers.