There's an event in the mobile app that also has exclusive on site rewards!
You earn the stamp by simply beginning the event in game (you will need to progress in the game far enough to unlock Ariel and build the Festival Plaza building, and will also need to complete any active quests you may have from Skippi). The cap is awarded for completing the event's introductory quest with Skippi. Neither gives a notification in game, but you will get a neomail on site when you earn them. As a reminder, this is a limited time event so these rewards will only be available until June 30th.