semipostal
noun
semi·post·al ˌse-mē-ˈpō-stᵊl 
ˌse-ˌmī-,
-mi-
: a postage stamp sold at a premium over its postal value especially for a humanitarian purpose
Example Sentences
Recent Examples on the Web The Save Vanishing Species semipostal was issued in 2011, the fourth in the series. 
Jeanette Steele, sandiegouniontribune.com, 2 Oct. 2017
Jeanette Steele, sandiegouniontribune.com, 2 Oct. 2017 Word History
First Known Use
1927, in the meaning defined above