In this article, we explain what an enclosure is and how to cite one, and we provide an example of an enclosure in a business letter. Or Enclosure if you enclose something with the letter.13. Enclosures. An enclosure can prevent a letter from becoming too detailed. Correctly format a business letter in block style with an enclosure notation. If you are enclosing materials with your business letter, you'll need an enclosure notiation. Learn more with our enclosure letter sample. Dear Mr. Fuller: Thank you for your recent order, dated May 9, 2013, for 100 boxes of business cards.