Hi Sandra,
There are obviously differences in e-mail sales messages and direct mail letters, but certain principles apply to both. Most direct mail sales letters are written to elicit a sale from the prospect right then and there, and so they must be carefully constructed to build trust and credibility. (Keep in mind that your sales message would be constructed a little differently if these are prospects you've already cultivated in some way.)
I've had extensive training in writing direct mail sales letters and not to write a novel here, your letter should consider these three rules:
- People like to buy but they don't like the idea of being sold.
2) People buy things for emotional, not rational, reasons.
3) Once they make the decision to buy, they need to justify their emotional decision with logic.
Also, there are two major ways to approach your prospect. Classical sellers focus mainly on the product or service with the goal of stimulating desire for it. Charasmatic sellers focus on themselves as well as the product or service. so they become part of what is bought by the customer. It would seem that in your industry, you would fall into the second category.
Here are some pointers:
Since you are not seeing your prospects face-to-face, you need to gather as much information as you can about your target market: age, social status. feelings, beliefs, desires, etc.
Know the primary emotions you are appealing to. Because emotions are the strong feelings that move people to action. There are many. Some will fit what you are selling and who your target customer is better than others: curiosity (will lead to interest, then inquiry); vanity; fear; optimism; happiness; pride; etc.
Develop a strong structure that leads the prospect along a path, where, at the end, you have overcome all their doubts and made them trust you enough to open their pocketbook. (Don't forget to use powerful headlines and start with a strong lead that delivers your big promise because most sales letters lose their prospects within the first paragraph). What's more, great copywriting, particularly for sales letters, has an invisible structure that holds it up and makes it work:
a. Identify a desire or fear of your prospect and hook them into the letter with it. Don't use logic at this point. It won't work.
b. Create a picture in his/her mind, the promise of what your product or service will do for him/her. How it will eliminate the fear or fulfill the desire. Now you have their attention.
c. Offer proof, perhaps a few bullets. Here is where you establish credibility. Facts, reliable sources of information, testimonials. Why your solution works.
d. Convey your Unique Selling Proposition. What makes you different? Why you are the best and only answer to their need?
e. Your reader should be convinced by now. So it's time to close and make your offer. Here you return to the big promise, link the product or service to the benefits/results, restate your USP, restate the offer, and ask for the sale. Of course, in the close, you also need to make your offer sound like a good deal, rationalize (again) the buying decision, add an unexpected benefit/bonus, provide some kind of guarantee, and create a sense of urgency (why buy now?).
There is a lot more to it, but these are some basic tips. Hope this helps.

