According to Stamp Collecting Round-Up :

Linn’s Stamp News reports that the United States Postal Service will no longer issue aerograms when the supply on hand is gone.

David Failor, executive director of stamps services for the Postal Service is quoted as saying, “Demand for these has been next to nothing for the past five years.”

An aerogram (or aerogramme) is a thin piece of foldable and gummed paper for writing a letter for transit via airmail. No enclosures are permitted in these letters, which can be sent internationally at a special rate.