Rocks with Personality

If there's no paper left, the sign itself may come in handy :o)

One problem in the logic flow: There is no actual solution for: IF the paper is out AND you have recently contributed. It assumes that those two things cannot exist at the same time. Obviously, it's saying that regardless of whether or not you've recently contributed toilet paper you should contribute more, but how is that fair?
Though I think in that case Nick's solution is appropriate.