Proposal letters are written documents used by businesses and entrepreneurs to outline an offer of action or service. They can be used for a variety of commercial activities, from bids for large-scale projects to requests for freelance services. A successful proposal letter should effectively introduce and sell your credentials and services in an engaging, yet professional manner. They should also be brief and to the point, making sure all pertinent information is included without being overly long-winded. Make sure to include compelling and relevant facts and figures to support your ideas, as well as convincing arguments and evidence that demonstrate why you are the best option for a given project.