Honestly, the second you finish reading this, you should be on OpenTable making dinner reservations (for whatever's left—you're already behind on this one), scrambling to the nearest chocolatier, and generally getting your ass in gear. If you think you're going to whip up something last minute and make it appear thoughtful, you're wrong. She will know.
Don't buy anything from the drugstore. Don't. It's not so much the lack of quality that gets you in trouble (though, yeah—that too), it's more the extreme lack of planning. Hypothetically, you could have just remembered it was Valentine's Day twenty minutes ago and dropped into the Duane Reade on the corner, wildly grabbing things from the shelves. There is nothing romantic about that.
The effort expended and the foresight behind it is everything.