It was up to the postal worker (who probably couldn't care less) to explain to her that she should go to the end of the line. I would probably have said "Excuse me, but I believe I'm next in line" rather loudly if she walked up to the window in front of me. But since I'm guilty of doing stupid things on occasion, I try not to get too bent out of shape at other people's stupidity.
Things like this are one more reason I love our little post office - the longest line I ever see there is three people!