Set Up Your Scene
Add a Mailbox component to the player, drag it into one of the example UI prefabs, and use a trigger to open it when needed.
The UI prefabs are in the Prefabs folders of the standard and Inventory examples.
Save letters
To keep letters after restarting the game, add Mailbox Memory to the Game Creator 2 Remember component. It saves the letters, read state, hidden state, and whether an item was claimed.