Postcards are easy to produce and relatively inexpensive to design and print.
Best yet, when you want to announce something to your clients at the last minute, postcards can be printed and mailed quickly. Among the various forms of direct mail formats, postcards are probably most appropriate for short messages. There really isn't enough room on a postcard to sell more than one idea at a time. If you plan a whole mailing campaign, each postcard will have its own message that reinforces the main theme of your campaign. For instance, you may want to convince your clients to use your firm because of your use of the latest technology.