A Promotional postal order ( or PPO for short) is a special type of postal order that is issued in the United Kingdom by various companies in conjunction with the Royal Mail to promote their goods and services. These have been issued at various times since the 1970's.
Michael Brill British Postal Orders, 1881-2005.
L.J. Clark A Catalogue of British Postal Orders 1881-1991
Howard Lunn Illustrated History of Promotional Postal Orders
Various issues of Postal Order News