My impression FWIW is that the ‘giving makes you happier’ point wasn’t/isn’t advanced to claim that the optimal portfolio for one’s personal happiness would include (e.g.) 10% of charitable donations (to effective causes), but that doing so isn’t such a ‘hit’ to one’s personal fulfilment as it appears at first glance. This is usually advanced in conjunction with the evidence on diminishing returns to money (i.e. even if you just lost—say − 10% of your income, if you’re a middle class person in a rich country, this isn’t a huge loss to your welfare—and given this evidence on the wellbeing benefits to giving, the impact is likely to be reduced further).
E.g. (and with apologies to the reader for inflicting my juvenilia upon them):
[Still being in the a high global wealth percentile post-giving] partly explains why I don’t feel poorly off or destitute. There are other parts. One is that giving generally makes you happier, and often more happier than buying things for yourself. Another is that I am fortunate in non-monetary respects: my biggest medical problem is dandruff, I have a loving family, a wide and interesting circle of friends, a fulfilling job, an e-reader which I can use to store (and occasionally read) the finest works of western literature, an internet connection I should use for better things than loitering on social media, and so on, and so on, and so on. I am blessed beyond all measure of desert.
So I don’t think that my giving has made me ‘worse off’. If you put a gun to my head and said, “Here’s the money you gave away back. You must spend it solely to further your own happiness”, I probably wouldn’t give it away: I guess a mix of holidays, savings, books, music and trips to the theatre might make me even happier (but who knows? people are bad at affective forecasting). But I’m pretty confident giving has made me happier compared to the case where I never had the money in the first place. So the downside looks like, “By giving, I have made myself even happier from an already very happy baseline, but foregone opportunities to give myself a larger happiness increment still”. This seems a trivial downside at worst, and not worth mentioning across the scales from the upside, which might be several lives saved, or a larger number of lives improved and horrible diseases prevented.
There are diminishing returns to money buying happiness, but it looks like they set in after pretty high incomes (starting at $95,000, and even higher if you live in a wealthy area).
So donating more on the margin when your total income is less than $95,000 USD seems to trade off directly against your happiness.
One can probably realize a lot of the egotistic benefit of donating by giving small amounts, e.g. $30 / month to GiveDirectly.