Thank them for the compliment. Assure them that any kind of feedback is useful and will be passed on to others.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
I want to thank you for the kind words you said about <company or employee name>. Your thoughts are important to us, but we're especially pleased when the feedback is positive. I'll be sure to share it with <people affected>.
We try hard to provide the best possible <service/products> to our customers and their satisfaction is our reward. The more satisfied customers we have, the more our business will thrive.
We really appreciate you taking the time to let us know we're doing something right. We look forward to serving you in the future.
encl: <List of enclosed items goes here>