So I've nearly finished reading "The Life You Can Save", by Peter Singer, in which he outlines his main argument and thought experiment in support of effective altruism as a moral obligation.

He claims that we are morally obliged to save a drowning child, even if we were to damage our trainers, as the life of a child far exceeds the price of trainers or a nice suit in value. He therefore concludes that, in the same manner, whenever we choose to buy something in which it would not nearly have as negative of an effect to not have than the death of a child, we act immorally through omission, exactly as we would in the thought experiment. Key things to consider are: immorality through omission, rather than only comission; and the idea that we are still obliged to intervene where we can, even if we did not cause or bring about the situation in which we must intervene.

I am not asking anyone to accept this argument, but, given that it is true, aren't the implications that supererogatory goods do not exist? Doesn't this make it impossible to do something "more than duty", if we are obliged to act in every way we can until if we acted more we would be sacrificing something as great or greater than that which we could prevent? In this utilitarian, calculable fashion? When I think of examples of supererogatory goods, charitable giving is the first that comes to mind. If Singer is right that philanthropy is in fact obligation, then doesn't this eliminate the possibility for supererogation?

Note: Effective altruism also holds that one is obliged to use time and money by giving to charities that will bring about the most good, so volunteering as an example of supererogation could arguably be considered going against the obligation of effective altruism, as your time could be spent on something that would bring about greater good or money that could be sent to the poor in the long run. (And yes, it is these implications that are somewhat leading me to reject Peters altruism as obligation theory. I have always believed that it is only wrong to cause something, or to not prevent something that was brought about by one's own fault).