Learn what letters to clients are, review tips to help you write them and follow these templates when crafting engaging client letters. Below is a sample sales letter and a sample letter of inquiry, each following the essential components of a business letter.Edit, sign, and share sample letter client online. No need to install software, just go to DocHub, and sign up instantly and for free. 1. Use a professional letterhead. The business letter will be a representation of your company. Here is a sample of customer complaint letter. We include 3 free samples of complaint letters. In this article, we explain how to write a business proposal letter and examine a few sample letters. A typical NYC co-op board application will require three to six personal and professional reference letters per applicant.