Pantheon: Rise of the Fallen, you are now officially too late. A forum post this week states that after internal developer discussion, the devs made the decision to remove the store in prep for a redesign of the interface and a re-evaluation of the various items up for sale.
Any orders already placed via the store will still be honored, so you don't need to worry if you had purchased something just before the shutdown. The post also makes note that if the store is brought back, no items with any sort of statistics or concrete advantages will be sold there, although there are no hard and fast decisions about when or whether it will be brought back. Of course, it's probably a little early to be worrying about that anyhow, but points for thinking ahead.