A proposal letter is a type of business cover letter attached along with a formal business proposal. It aims to persuade and convince the reader to accept the proposal presented in detail.
It is best to begin your proposal letter by introducing yourself and your company. Then set forth to explain how you can best tackle the problem. It is also good to give an overview of the proposal, by giving a gist of the most important information including estimated cost and time-frame.
Though your aim is to convince the reader about your proposal, make the reader/the company your focus. Elaborate how the suggested proposal would benefit them. Back it up with researched facts. Quote or give references of similar work you have undertaken.
Use only proven facts and figures as there is no place for flowery language and false promises here. The letter must sound sincere and convincing without sounding fake and conniving.
As you are concluding the letter offer to fix up an appointment for personal meeting. This would help pursue the idea in person, and the reader would be persuaded to take action.