Back at the start of the week, a special 'Undertale Pack Bargain Set' was made available for Groove Coaster: Wai Wai Party!!!! in Japan. That same pack has now see release around the rest of the world.
The Undertale DLC pack includes 10x UNDERTALE songs, 2x navigators (Papyrus and Sans), and 6x avatars (Human, Flowey, Soul, Napsta Blook, Temmie, and Annoying Dog). You can get the pack itself for $14.99, or you can buy Groove Coaster: Wai Wai Party!!!! and the Undertale DLC pack in a $59.99 bundle. This deal will be offered until Oct. 4th, 2020.