My macro (schematically) looks like this:
/castrandom [flyable,btn:2]Sandstone Drake;[flyable]other fly mounts;[btn:2]Chopper;other ground mounts
so I can directly summon the "taxi" mounts with right click.
Taryble, I have no real idea what's happening there. Have you tried it without any Addon enabled that could interfere with the keys (mainly Domino^^)?