Thank them for their interest and give them a potential next step.
1234, Main Street
Boston, MA 02123
09 April, 2005
<Recipient Address Goes Here>
Thank you for your interest in <product or company name>. I've enclosed the information you requested and would be happy to answer any questions you might have.
Abc Company is dedicated to <service or product>. <Company's credentials and product's attributes.>
If you would like to discuss your specific needs please feel free to call us at <phone number>. We really appreciate your interest in Abc Company and would like to help in any way possible.
encl: <List of enclosed items goes here>