Can give the letter to someone who sees the future and leave it in a visible location. Some future visions begin with waking up from bed. Then the person might have a future vision about it and getting curious about the letter when seeing it for the first time. I felt the sensation of pulling hair on my arm for the first time in a future vision, so finding a new letter where the mailman cannot reach would get all curiosity. If it is to affect yourself, you cannot change your own direct past, because you only sent it because you did not get it. An alternative timeline might however be derived from ours, where you don't send it because you got it.