This letter praises a company for a superior product.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
I recently purchased <product purchased> from your company and I thought I should write and let you know how pleased I am with it.
In an age when so many products don't live up to their advertising claims, your <product purchased> does everything you say it does. I am one satisfied customer, and will continue to be for years to come. Thank you for your time.