The most evident answer is that Edie's life would have fundamentally changed had Chris followed through and sent that letter. Edie was in a position of waiting, something upon which she herself remarked. Edie argues that there are millions of women “waiting by mailboxes for one letter or another. Edie suggests that she “was never made to go on like that.” This means that if she had received the letter from Chris, she probably would have acted upon it. Given the fact that she was waiting for so long, it makes sense that if the letter had come through, she would have taken it and seized upon the opportunity that presented itself to her. Since she had already established that she would not be one steeped in waiting, it becomes clear that had Chris sent her the letter, she would have acted upon it.
This action would have changed the course of her life with the mailman and the life they made with a family. Edie had made up her mind that she was not going to spend her life waiting. Action was to be taken. When the opportunity with the mailman presents itself, Edie takes advantage of it and seizes upon it. This means that Edie was going to take action. Had Chris sent the letter that Edie coveted, action would have been taken in this realm. Since this does not arrive, Edie takes action in terms of recognizing the opportunity that presents itself with the mailman.