Saying people's applications are 'Pre-approved' will do the same. Check for comprehension. Get somebody else to read the letter and the order form and ask them if they know what to do. The person you choose should not know much about your business but ideally should be a prospect, who has never seen the letter before. Get them to read it aloud.
People read things out loud in their heads.
In so doing they sometimes stumble over words or phrases. Someone unfamiliar with the subject is far more likely to make the mistakes the recipient would than you are. Ask if they have any questions about the product or what has been written. Those are likely to fill in any gaps in your story. Also, encourage them to ring you to ask questions.