Learn how to write an invoice cover letter to remind customers of their payment due and offer discounts or benefits. See a sample letter and email, Cover Letter for Invoice. Latest revision. Formats Word and PDF. page. Complete the template. This invoice cover letter is intended for use by a business, whether a sole trader, corporation, or, How to Write an Accounts Payable Cover Letter. Here are seven steps you can follow when writing your accounts payable cover letter: 1. Research the position and company. Before you start writing your letter, it can be helpful to research the company and the position so you can adjust your letter accordingly.