A piece of postal stationery is a stationery item, such as a stamped envelope,letter sheet,post card,letter card,air mail ,aerogram or wrapper, with an imprinted stamp or inscription indicating that a specific rate of postage or related service has been prepaid. It does not, however, include any postcard without a pre-printed stamp. In general, postal stationery is handled similarly to postage stamps; sold from post offices either at the face value of the printed postage or, more likely, with a surcharge to cover the additional cost of the stationery. It can take the form of an official mail issue produced only for the use of government departments. It can be an issue of a military force where an army, perhaps in a distant war, issues letter sheets for the use of its troops to write home. Postal stationery can be overprinted by the government or, occasionally, by a private overprint. In emergency situations, postal stationery has been produced by handstamping envelopes with modified canceling devices; many of the rare Confederate postmasters’ provisionals are of this form. Finally, some postal stationery can be printed to private order. In this last case, stamped stationery bearing indicia is applied with postal administration approval and with specified regulations, to paper or cards provided by private persons or organizations. Private impressions result in a wider range of denominations and designs compared with governmental issues.
Aerograms:The postal services of some countries also offer a form of letter sheet called an aerogram consisting of a blank sheet of paper with folding instructions and adhesive flaps that becomes its own envelope, and carries prepaid postage at either the international airmail letter rate or at a special lower aerogram rate. Letter sheets lend themselves to airmail usage because they are lightweight. Enclosures are not permitted in aerograms. Sales of aerograms in the United States ended in 2006 due to poor sales.
Letter cards: A letter card almost has the advantages of a postal card as far as weight and size, but also the advantage of privacy of contents is concerned. It is a double card, folded over, with gum or adhesive applied to the three open edges. It is then opened by the recipient by tearing perforations on the three sides that are on the message side of the gum.The gummed strip around the card is then discarded, giving rise to the problems collectors have in finding intact used cards. The US has never issued any letter cards.
Letter sheets: Before 1845 correspondence was not enclosed in an envelope. Letters were folded, sealed, addressed and postmarked on the outside. This continued even after adhesive postage stamps were introduced. The popularity of folded letters led postal authorities to introduce stamped letter sheets. These became available in the U.S. in 1861, but the first official postal stationery were the 1838 embossed letter sheets of New South Wales. These were followed by the Mulready stationary that was issued by Great Britain at the same time as the Penny Black in 1840. Since then, most postal services have issued a steady stream of stationery alongside stamps. Often the design of the stationery mimics the contemporaneous stamps, though with less variety and lower printing quality, due to the limitations of printing directly onto the envelope. Much later, 1947 in the U.S., letter sheets morphed into lithographed air letter sheets or aerograms.9Source-wikipedia)