To be able to convince somebody by your letter, it might be wrote and read easily.

Use short sentences. Leave empty space between them. Like this.

I often see a very dramatic mistake in my colleague’s letter.
They merge all the text in one paragraph trying to build complicated sentences to make em seem smarter.
Ha ha. Emotions convince. Not Logic.

Nobody will read what seems to provide a head ache at least)
Besides, as it proven, active verbs used in a written way, work and affect like a real boss.

“Do you like our product? Get up and call 555-555-55 right now!”

The thing is, until you say to you potential customer what he should do – he wouldn’t! It is really proved by lot of ad which didn’t contain this addition and that ones which contained! (as a comparison).

One more advice to you letter. USE references!
References from satisfied clients are the most powerful tool of your letter or web site or whatever.
If you don’t have real references – write it by yourself on behalf of friends of acquaintances. Yes, it’s a little trick, but it works and brings the money. As soon as you’ll get references from real clients – you are free to put it with your letter of offer everywhere you wish!