In store only but you can click and collect to check stock save a wasted scouting trip. Pretty sure this is as cheap as its ever been. Last posted 2 weeks ago at £16.99 its even better value now!
There's been a few successful mash up titles in recent years on nintendo systems Persona Q on the 3ds springs to mind and this i'm happy to say is another. (imho, imho, imho. There we are, covered myself)
Smushing the Mario & Luigi rpg series and the Paper Mario rpg series together could easily have ended in tears but its a solid game. Not spectacular, but definately worth a play.