The discount will have more impact if it has a time limit on it. Be specific about the terms and conditions of the discount to avoid any confusion.
1234, Main Street
Boston, MA 02123
08 April, 2005
<Recipient Address Goes Here>
For <duration of discount> we're offering <discount>.
If you would like to take advantage of this offer, simply <instructions>.
Don't miss this opportunity. We look forward to hearing from you.
encl: <List of enclosed items goes here>