Psychological egoism is defined to mean agents can only serve their own preferences.
The proof is simple.
A preference is what an agent will decide to pursue, given the option (or if exclusiveness is forced on them). If an agent can pick up a red ball or a blue cube, it will pick up the one it prefers. (Test: go look at the Oxford or Webster dictionary.)
To give to charity is to prefer someone else's material interest over your own. It is not to be selfless. It is to prefer moral or social interest over material interest. It is e.g. to enjoy having someone else buy with the money rather than to enjoying buying with it yourself.
Psychological egoism is a straightforward consequence of the nature of discrete agents with preferences.