The phrase "an extra item" is correct and usable in written English.
It can be used when referring to an additional object or element that is not part of the main set or list.
Example: "We decided to include an extra item in the package to enhance the customer experience."
Alternatives: "an additional item" or "a supplementary item".
They considered the act of smuggling an extra item into the express lane tantamount to high treason.
Originally, catalogs took the same form as the books they listed; being made of the same material, the catalog was an extra item of the collection itself.
Powell's Books, for example, offers a subscription service through which it chooses a new book and includes an extra item like a related book or candy — personalized touches that it says big sites can't match.
When you plug in your iPod to the dock connector, an extra item called "Speakers" appears in the main menu.
You may be getting an extra item for half the price, but you are still spending more than you planned to.
For a merchant to convince you to add an extra item to your shopping cart, or buy an appetizer with your meal, the interaction must happen sooner.
Face validity and content validity were acceptable: neither patients (n = 152) nor psychologists (n = 3) or a hepatologist suggested any extra items to be included.
The few items I did bring (including a toothbrush, an iPod, and a few extra items of clothing) were tucked away in my pockets.
The few items he does bring (including a toothbrush, an iPod, and a few extra items of clothing) will be tucked away in his pockets".
The broadband privacy rule, among other things, expanded an existing rule by defining a few extra items as personal information, such as browsing history.
Adams, a versatile infielder, decided to squeeze a few extra items in his suitcase, just in case.
