You. IDIOT.
Splatsville: Leaving Grizzco, head down the stairs, and take a left at Ammo Knights. The postbox is nearby, around Hotlantis.
Inkopolis Square: It's by the story mode. For context, next to Ammo Knights.
Inkopolis Plaza: It's in front of the tower. Literally in your line of sight when you hop off the train.
