The letter that already exists.
Let me walk through what actually happens when you use this product. You answer five questions: vocation, values, what puts you in flow, the trajectory you're probably on, the threads you haven't resolved. You look at the timeline. You find a node that means something. You write.
The postcard enters the system. Sometime later, it comes back. Not immediately. Your future self has been there the whole time. They just needed the time to write. Some postcards get lost. This is not a bug.
The three branches produce three qualitatively different emotional experiences. Probable returns you to the weight of accumulated choices. Preferable returns you to the particular quiet of having made the harder decision and finding it was right. Possible returns you slightly disoriented, with the quality of mild wonder that belongs to someone who took the road that should not have worked.
If any of this comes to pass, it will be because you made it so.
Not because a poem told you it would.