Ah, an interesting question. Remember that the story says, " The white of two letters in her lap grew indistinct. One was to Harry; the other was to her father."
It doesn't say that she mailed them, or even that she finished them. Therefore, given the rest of the story, I think the letter is two things. First, it includes a lot of standard family news and greetings, because she's not bold enough to move to the heart of things right away. Second, I think it tells her father that she's going away with Frank…and that it never gets sent. I think it sits around in a drawer like Eveline herself, gathering dust.