The phrase "an extra box" is correct and usable in written English.
It can be used when referring to an additional box that is not part of the original set or quantity.
Example: "We need to order an extra box of supplies for the event to ensure we have enough for everyone."
Alternatives: "an additional box" or "a supplementary box".
Especially when the cops would rather not check an extra box.
One officer even gave him an extra box of corn flakes and an extra orange.
This adds an extra box plus wiring, which you may not want.
Mr. Britt noted that he felt smart TVs — Internet-connected televisions that include computer chips and software — would be better than requiring an extra box to view Web content.
The BlackBerry system, for example, can be set up in a few hours; it simply involves installing an extra box that establishes a secure link between the wireless operator's systems and the client company's e-mail server.
Many were never told that they had to mark an extra box on their ballots to be counted, and some were erroneously told that they were not allowed to vote for a Democratic candidate.
The Company figured they might as well make up a few extra boxes, just in case there were a few Northerners who might thing a Frigidaire was a bureau.
While you're packing, keep a few extra boxes for things you plan to put in the yard sale or just in fact throw into the garbage bin.
When you do this, make sure you have a few extra boxes in which to put items of different categories, and a trash can handy for any trash that may have accidentally gotten thrown in with your mess.
And it never hurts to add an extra litter box, or three.
Census workers are being taught, for example, to look for clues that a house has two families living in it, such as an extra letter box or electricity meter.
