Instead, write your letters as though you are trying to help them with a need as opposed to selling something. Recognize, that means making no mention of your prices or direct mention of your service. Instead, identify something the prospect is doing and mention how you may be able to help them do it more efficiently. For example, "I noticed you host many fashion shows to publicize your designs. Based on the work I've done for x company , I believe I can help you reach out to the community faster and more effectively. Would you like to chat sometime in the near future?"
Writing in this way does several things. A.) Personalizes the letter so the readers see that you took the time to know them B.) Identifies your credentials C. Suggests a course of action without being too pushy.
Potential clients tend to respond favorably to letters formatted this way because you treat them like people as opposed to another sales figure. Most importantly, it gives the impression that you care by identifying a need and offering to fix it. And who wouldn't want to work with someone who actually cares?