Can I possibly have a hint about where to find the person to deliver the letter to? I've explored 7 islands (excluding home), but still haven't been able to complete the quest.
Wow, congrats, having explored all 8 islands means you've basically made it through the game! How many souvenirs did you find?
Without spoiling too much, there is a short outro sequence that somewhat resolves things. It triggers after you've visited (or flown past) all islands during a single journey. I suppose this is a bit difficult to achieve right now because the game doesn't save your progress yet. Good news is, we're working hard on implementing save games and have already made good progress on it!