I'd just buy it from Amazon or someplace like it. They have it $10 cheaper than the SE Store anyway. (The SE store never seems to discount older items. Stormblood is still selling there for the full price it was at when released, so now you're better off going to a third party retailer so long as it's someplace you trust.)

p.s. Though as Ekinoxe mentioned, that's not possible if you have the Steam client. In that case, expansions can only be purchased through Steam.