For commercial artists, particularly freelancers, promotional work has to concentrate on cashflow.
Being in the promotional business yourself, you'll realize that your potential clients are all promotional businesses.
So whatever you send them as a promotional letter has to be top quality. You're selling to experts.
In your case, your promo letter has to target clear areas of interest for the specific addressee.
This is a bit like bringing people your portfolio when going for a job, but you have to hit your targets.
Hard copy is inefficient.
You can achieve more with electronic media, even if your work is hard copy.
It's best to send your letter to clients as an email, with embedded media or file attachments.
Find out how they want to see your media. Do NOT send attachments, etc, to people who specifically say they don't want attachments.
The example here is a personalized email from a digital graphic artist who has made contact with an advertising firm. The ad agency is interested in graphic work from freelancers, and has said they'll accept attached media.
Business letterheadAttention: Mr. Pic Checker
Dear Mr. Checker
I refer to our phone conversation yesterday regarding possible freelance work with your agency. Thank you so much for your time and explanation of the work involved, it is very much appreciated.
As requested, please find attached:
If you should require any additional information, please contact me on the above phone number or email.