In this article, we discuss how to write a business proposal letter and we offer sample letters and templates you can use as a guide. Learn what letters to clients are, review tips to help you write them and follow these templates when crafting engaging client letters.Sample Customer Letter of Recommendation. In this article, we discuss how to write a demand letter, what to include in a demand letter, and provide a Texas demand letter template. Sample Letter 7.30 thanks a customer for continued business support. Sample Letter 7.31 thanks a customer for repeat business. A letter is important. It puts your complaint on record with the company, helps preserve any legal rights you may have in the situation. This e-book provides templates for 21 most effective credit and collection letters that will help you communicate better with your customer.