Just because you think it's wonderful, that doesn't mean to say your reader does. You're writing to somebody who has never seen you and doesn't know you and is probably fairly sceptical, so make sure that you have adequate proof and testimonials. And, perhaps the biggest mistake of all is not to ask insistently enough for the order. It seems an awful shame to spend time, effort and energy writing a persuasive letter without asking for the order as strongly as possible. This means re-emphasising the benefits that will be obtained, underlining what will be missed if the reader doesn't reply, reminding that reader of any incentive you're offering, and of course making it easy to reply.
Big shots are only little shots who keep shooting.