Hey Rebecca, I am from Buenos Aires, Argentina, and YES, you can say "la posta" to refer a person, it is like if you say "this person rules". It is not common, it sounds strange sometimes because "la posta" started as a way to say "the whole truth about this is..." = "la posta es...". I can imagine a possible scenario to use "la posta" to refer to a person: if you are talking to a friend about 3 persons (Juan, Pedro, José) and your friend is not sure about the intentions of them, then you can say this to your friend: "La posta es Juan, él dice la verdad, los otros dos (Pedro y José) mienten".

Hola Rebecca, "la posta" se refiere a la verdad. Tengo "la posta" significaría: sé la verdad sobre algo, sé lo que realmente pasó. Por lo tanto no se aplicaría a una persona sino a un acontecimiento. "La posta" refers to the truth. I have "la posta" would mean: I know the truth about something, I know what really happened. Therefore it would not apply to a person but to an event.