A survey done by the United States Postal Service found that using repositionable postal notes could raise response rates up to 37%!
 

Repositionable Postal Notes (RPN), or sticky notes, are friendly attention-getters, placed on envelopes, letters, newspapers and packages and can be sent through the mail. Our RPN's are approved by the USPS for use on the outside of mail envelopes, cards and magazines. RPN’s can also be used for the inside of an envelope and are not restricted by postal specifications. The message on the note could identify dates, phone numbers, or personnel, so the recipient is able to remove it from the mail piece and place it at their work area or in another high traffic location. As a result, this could increase the size of your audience far more than conventional labels ever would.