I managed to solve the shrines puzzle indeed! But for the mailboxes I really have no clues. I noticed one has a dreamcatcher on it, and there's indeed a dreamcatcher in one of the save rooms, the one in the wagon, but I can't interact with it in any way. On another mailbox there's a lantern, but it's full of lanterns everywhere. As for the bell, I think I didn't see any around. I am definitely stuck on that...
Viewing post in Hope Springs: Penstroke comments
So to give you a little more to go on, there's essentially three envelopes just hidden throughout the game. They can be hidden anywhere from slightly sticking out from behind something on a wall to floating I'm something else. Sorry for being a bit cryptic but don't want to spoil it too much for others. Hope that helps. :-)