The Store & Inventory is a single window with two tabs. If you lack an item in your Inventory, just switch to the next tab to buy it in the Store. Almost all game items found in the Inventory can be both purchased and obtained in the game.
Items that are activated from the inventory can be filtered by checking the "Usable" checkbox.
The New ribbon indicates newly received items. They can also be filtered by checking the corresponding checkbox.
At the bottom of the window there is a search field that allows you to find an item by name.