pre-seeded the boxes before I even knew who would be the one choosing

If I met someone in real life who was doing this trick (at least before I started spilling my opinions to the universe through my comments to this blog), I would strongly suspect that you were doing exactly this. And then I would definitely pick both boxes. (Well, first I'd try to figure out if you're likely to offer me any more games, and I'd pick two boxes if I was fairly confident that you would not.) And I would get all of the money, since you would have predicted that I would pick only one box (assuming that you really seed them based on your honest best prediction).

On the other hand, if the situation is not presented as a game (even when I still don't expect any iteration), I pretty consistently cooperate on all of the standard examples (prisoner's dilemma, etc). But since feeling like a moral and cooperative person (except when playing games, of course) has high utility for me, I'm not really playing prisoner's dilemma (etc) after all, so never mind.