How can we just chose one charity?

In the essay Peter Singer makes the claim that if everyone in his position donated  5$ to a charity, in his case to the Eastern Bengali refugees, then the issue could be avoided and resolved. I believe that this approach, while nice sounding, over simplifies the issues at hand. We are presented with so many different people in poor situations who need aid. There are charities for rape crisis, orphans without homes, cancer victims, animal abuse, and so many more. If one just had to donate 5$ to a charity how would they chose which is deserving of said amount? This puts everyone in a difficult situation. For example, I shaved my head for a cancer charity in March. I raised over 3,000$, I donated a significant amount to my own cause. Later I was unable to donate to other foundations because I had already donated to the one I was funding for. With so many things out there that need aid, it is seemingly impossible to make a big enough difference in any particular one. For arguments sake let’s say that everyone could donate 5$ to a charity, if everyone donated to the cause they thought was most important would it really make any progress? In Singers essay in order to make a difference we would all have to donate to the same organization which is not plausible given the personal reasons why people donate. Singer must face the reality that there is thousands and thousands of things that need aid, it is easy to take the pessimistic position that we should not even bother donating to one.