One thing to think about is that anything is better than nothing. Any way you look at it, when you waste time thinking about box toppers, you are missing opportunities. The trick, then, is to start box topping, not spend time looking for the best box topper.
Who knows, your market might like something printed on pink paper in one color ink. Some markets prefer 4 color ads. Yours might have better results with $ off over free ad ons.
Get things going, give yourself permission to screw up, and start making some mistakes. Knowing what not to try is almost as profitable as knowing what to try.