This fundamentally is about a need to communicate or to receive communication. It might even at times show mind to mind communication, or your intuitive faculty.
If you are waiting for money it could be about that. But many dreams in my database mention post office as a background for other events. So it might be worthwhile reading Settings in Dreams.
Post office can be a meeting place, or even a place for flirting. For some people it is a place of decision, where you wonder whether to communicate the thoughts and feeling you feel.