The most widely used business letter formats are full block and modified block. How do you write a business proposal letter?Here are a few example proposal letters to inspire you as you create your own. You can use the cover letter to showcase key wins, client stories, or key features that make your company stand out. Call it a business proposal letter and it's a short, 1 page message to your potential client. It doesn't change the fundamental situation. Proposal Cover Letter. A business proposal letter is a document sent to a prospective client or partner, introducing your services or requesting an opportunity. Each of these business proposal templates is fully customizable, so you can change the text and colors. Highlighting this in the introduction will catch their attention, allowing you to explain further in the rest of your proposal.