Hello

I finished the game and unlocked the flower mask, the monster mask and found objects like the postcard and the monster plush. The problem is that I didn't understand what it can be used for, except for the hateful side. When we leave the cave at the beginning of the game the ghost tells us to send him a postcard but we can't put anything in the mailboxes... Anyway, if someone has discovered the secrets, I'm in for it. Thanks

According to https://steamcommunity.com/app/572890/discussions/0/1779388024834265309/#c1771511442692609481 there's no use for the postcard.