A second letter enclosed in a mailing package to stress a specific selling point. Its aim is to lift (increase) response. 

Usually reinforces the message of the main letter or present an argument for buying from a different point of view. Often signed by a celebrity or other influential individual.

For example, it may be a note signed by the publisher and enclosed in a small envelope saying, “If you’ve decided not to order, please read this.” This is a common technique that does not add significantly to the cost of the mailing.

Synonyms: pub note, publisher's letter