Jasminocereus thouarsii var delicatus from 1998 illustrates a cactus of the Galapagos Islands. I'd say the only time a specie of that genus is on a stamp.
Gypsophila paniculata from 1998 again.
Befaria resinosa from a 1986 series called FLORES: costa, sierra y oriente. On the stamp: Payana Mutis Sierra. The genus is also called Bejaria.