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The first mailbox you find is on the island directly to the north of the queue sign, but you can’t do this yet. You have to wait until you reach the second one, which is directly east of the island with the inflatable flamingo. Simply walk up to the mailbox like you would with any exhibit, and a fast-travel-like system will open up allowing you to teleport between mailboxes. Use this system at least once to get the achievement.