These mailers are light in weight and are made of molded polystyrene foam plastic. One to ten 1 x 3"" slides can be held in individual slots. Plug-in lid. Mailers are designed to stop slide breakage in Post Office cancellation machines. Between foam unit and optional cardboard mailing sleeve is a space to place report forms. Size is 4""L x 2""W x 2.5""H. Weight 1/4 ounce.