I think what I'm asking isn't actually possible, because while I can apply my credits to my total in the cart and it updates to show that my new total cost is $0.00, when I proceed to actually pay it's still showing that I need to pay $50. This sort of makes sense to not allow gift card purchases with store credits, but the site is a little buggy in making it look like it should work. If I'm overlooking something, please let me know.