 Advertising postcards may be mailed or distributed in other ways.An advertising postcard is a privately, commercially produced, rectangular piece of stiff paper (typically 3.5 X 5.5 inches, or 148mm x 105mm in Europe) printed in a form that is easy to send through the post and is designed to carry promotional messages of products or services.From the 18th century, trade cards were used by businesses to promote a wide variety of goods and services.