It is so difficult to know where I should begin, especially when, unlike you, I already know the ending. But let us say that this story began with end of another.
When the mailman delivers mail to Ronnie Bostock's mailbox,he raises the mailbox flag, presumably to signal to the resident that mail has been delivered. (Ronnie's girlfriend, seeing the mailbox flag has been raised, seems to interpret the signal accordingly.) Although it may be the convention for mail delivery wherever the director/writer is from, it is not the case on Long Island, where it is the custom for the resident to raise the mailbox flag to alert the mailman that mail is in the mailbox waiting to be picked up. Once the mail has been picked up, the mailman lowers the flag - the opposite of what occurred in the film.
$78,151 8 March 1998