Are you purchasing a physical gift card or an e-gift card? They sell both on the site, so you have to make sure you specifically select an e-gift card. I haven't had any issues with purchasing with a Canadian address. I don't have a US credit card linked to my paypal, nor a US-based USD account, which is why I do it this way instead. I do request when I get sent payment from a renter, that they select that I receive funds in USD. Like
@DougEMG stated above, PayPal automatically converts payment to your country's currency, so renters have to specifically select for me to receive USD. Then I just use my PayPal balance when checking out on the Shop Disney site.