Hi Erica,
The "store item" option should be used if you are submitting a listing to your eBay store.
All of your active listings will show up in your Auctiva store, whether they're fixed price listings, auctions, or eBay store items=)
Having an eBay store allows you to take advantage of lower listing fees and longer durations for fixed price listings.
I hope this clears things up=)
-Mike