They are know as Postal Stationery and are highly collectable. It is a specialized area and can be found in Scott USA section of the catalogue.
Wow some fantastic ones here and the airmail envelope on this pic is still used with the exact same colouring etc as it was in the old days! I like the superman one and do you know the year it was printed?
The Superman postal cards were printed in 2013. At the same time Canada Post also issued self-adhesive booklets of 10 with those same 5 designs (and 5 different booklet covers), a water-activated souvenir sheet, and a self-adhesive coil with Superman's S-logo.
Although not all would prefer self adhesive type of stamps, still it could be a nice addition. I do have a Hello Kitty self adhesive stamp when I was in grade school which was almost torn when I tried to remove it from the paper.
I cant find my airmail envelope! I HAVE LOOKED everywhere for it and sadly they have changed the look of it now. I am so upset and will keep looking but that one is great!