air letter 
noun
A sheet of light paper folded and sealed to form a letter for sending by airmail.He always wrote on pale blue aerogramme paper (air letter sheets), the kind where you fold and lick the tabs on the top and sides, transforming them into simple but elegant envelopes....
- We are grateful to the people for the interest they showed in mailing air letters during this week.
- This was even more likely before fellow collectors started to pay attention to formula air letter sheets.