The Shop Key is a key item that appears in Paper Mario: The Thousand-Year Door.
The Shop Key can be found on Twilight Trail behind a log blocking the way. The only way to get the Shop Key at that point is to use Koops. After retrieving the Shop Key, Mario can then unlock the door in the Twilight Town Item Shop to get into the storage room where a Black Chest lurks. Unlocking it with the Black Key (that is also found on Twilight Trail but in a small shed) will allow Mario to use the Paper Tube ability.