compliments slip 
(also compliment slip)
noun British
A small piece of paper on which a company’s name, address, and logo are printed and which is sent out with goods or information, typically in place of a covering letter.These would get him into the Guinness Book of Records as collecting the largest number of compliments slips....
- Previous lots have included a pound coin, an e-bay compliment slip, a McDonald's Happy Meal Snoopy toy, used gift wrapping paper bought from Buckingham Palace gift shop and broken plastic beads.
- It is thought the chain loosely stems from a national appeal in the 1980s when people were asked to send compliment slips and get well messages to a boy who was seriously ill with cancer.