It doesn’t seem that way, sadly. I’ve only seen one report of someone getting Shauna this way and I suspect now that the person in question was trolling.

The Mystery Love Boxes only seem to contain Valentine’s decorations or hearts – nothing else. Which means if you want Shauna before the Valentine’s Day event ends, you’ll have to buy her with donuts (as there’s no guarantee she’ll hang around afterwards like Barney or Sideshow Mel did after the Christmas event).

I’ve written a post on this so take a look here if you need some buying advice.