It's surprising to see no Interac support, since Shopify are from Canada. I don't see any store here using this unless it supports Interac debit cards. I use a Square card reader with my iPhone at a merch booth at concerts, which doesn't need the full POS components, but more storefronts here support Interac than credit cards and you would be turning away a lot of business in a storefront context by not supporting it.