That "thing" in the ad is horrid... about as innocent looking as Al Capone.
Truth is, you don't need a "knife" per ce... a nice wood envelope opener for an example, works well... there are many envelope opener type items that are practical that can be used.
I've not seen much of anything on the market that offers an innocent look and proper application. You might want to experiment with a Swiss Army Knife or similar item... it might mean adding a fold to your billets, but that's perfectly fine, I tend to do this so as to take away from the idea that I might have some means of looking into the paper (and I've heard people say just that).
Though I've rarely used such a method, when I have I used a simple butter knife from the utensil tray.
