There are a lot of different kinds of letters. A very successful sales letter that I've used in countless industries is what I call the assumptive letter. The assumptive letter is based upon an assumption that people reading the letter will think you are personally writing to them. It will start with an assumptive statement:
"I know you've been contemplating (or considering or reviewing or evaluating or thinking about) buying a (such and such) or replacing (such and such) and moving up to a more advantageous position in your business or professional life."
"I share the awareness of what a positive impact or what a difference, or how much of an improvement or savings or enrichment doing that can mean to you. That's why I'm writing this letter."
"I've made arrangements for you to (and then you fill in either check out, try out, preview, acquire, purchase, sample, audition, experience, or whatever is appropriate) that in either your home or office without risk or obligation. Why am I doing this? One simple reason:"
"In my opinion, you can't possibly know the benefits, experience, exhilaration, enrichment, security, peace of mind, productivity (whatever it is) you can bring to your life through _______ unless you experience it."
"I don't think you should have to decide anything until you first experience for yourself how meaningful or beneficial ________ will be.
So, I want you to let me take the risk for a change. That's right! Everybody else says, 'Buy from me. Give us your money,' etc. I'm saying, do the opposite. Let me allow you to experience, to evaluate, to check out, to preview the product before you buy!"