You might have noticed a strange golden circular symbol on your Celestial Edition Starter, or Booster Packs... this is "Koku" representing the fictional money of Rokugan.  After you buy an L5R product, you can save the packaging to purchase from a list of promos available through the "Imperial Assembly."  The Imperial Assembly is the L5R fan club, which when you become a member not only gives you a quarterly L5R magazine, but allows you to vote on clan related storyline options, access to exclusive promos and events, etc. I'm pretty sure that you can trade approximately 20 koku for a rare exclusive card.  AEG is also running a promotion where membership is currently only $25 instead of $30.

Even if you aren't a member of the Imperial Assembly.. you might even be able to trade or sell your Koku to other players.  So remember. to save your Koku!