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I’m not sure that doing something that only directly benefits other people but makes you feel better about yourself as you’ve done something good (or less bad as you’ve not spent the money on something you’d have felt guilty about) isn’t in your self-interest. Other kinds of making yourself feel good count.
Hence why they like to specify “rational” self interest.
It’s rational to make yourself feel more good. That’s the final outcome of every aspect of self-interest that isn’t solely to remain alive. If the intention is to act solely in the self-interest of an emotionless unfeeling human-shaped robot: