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. Introduce yourself and provide background information.State your purpose for the proposal. Vendors should submit written proposals in the manner described in the specific documents for open requests posted on this page, below. The County of Santa Clara (County) is issuing this Request for Proposals and Financial Plans. Santa Clara St. San Jose, CA 95113. A business proposal letter is a document sent to a prospective client or partner, introducing your services or requesting an opportunity. We're much more than fill-in-the-blank forms. Any Worksheets and Administrative Requirements must be filled out as directed and returned complete, in the order received. Filing Period and Application Procedure Final Filing Date: Open until filled .