Everyone who is super are friends. That's really all there is to it. Superman, Korra, Cloak & Dagger? Sure. Ghost Rider, the Dragonborn, Huckle Cat? Why not?
Superfriends is a set of six 4" x 6" prints and was prepared special for SPX. They work great in those obnoxious 6-frame sets, which you can usually find on the cheap at TJ Maxx.
Shipping is included in price. Maybe if you're overseas think of throwing in a couple extra bucks.