Altruism is no longer valuable in evolutionary terms, but who cares?.

What's important to us as people isn't evolutionary value, it's satisfying personal preferences derived causally but not normatively from evolutionary value. Having money is one of those preferences. Being a nice person is another one of those preferences. If you deliberately make a choice that satisfies weaker preferences than the alternative, you're just throwing away utility for no reason.

If you haven't already, I suggest reading the Evolution Sequence, especially Alien God, Adaption-Executors, and Evolutionary Psychology, and the Metaethics Sequence

Now, if you want to make this into a reaaaallly repulsive ethical dilemma, add that true utilitarians should refuse to tip so they can donate that money to a charity that produces greater utility than the tip does.