The special offer will have more impact if it is truly "special," i.e., it has a time limit on it. Be specific about the terms and conditions of the special.
1234, Main Street
Boston, MA 02123
08 April, 2005
<Recipient Address Goes Here>
For <period of time> we're offering <special offer>.
Since this special offer is only valid <how long>, please act now. We look forward to seeing you!
encl: <List of enclosed items goes here>