Currently there are a few perks available for SMASH in the XP Shop. These perks are particle trails and knockout sounds that other players will see and hear. You can change your active trail or sound with the signs in the Lobby or in the spawnbox of every map. You can also buy a party pass, this allows you to create party matches. (You also have a Party Pass with Silver, Gold, Platinum, Emerald or Obsidian.)
These change the particle trail that you leave behind when you move. The following 20 trails are available.
A video demonstration of some of the trails can be found here
These change the knockout sound that will be heard when you kill a player. If you are thinking about buying a sound, there are signs in the Lobby which let you hear the sounds first. The following 17 sounds are available.
The SMASH party pass allows you to create a party and custom matches. You will be able to rotate maps without leaving the match, modifiy the number of lives and the item spawn chances with the Item Editor. More customization options will come in future updates and will not require any additional purchase to unlock. The liftetime pass is available for 20,000xp in the XP Shop.