You buy the licence for 1000 tokens, then enlist to the next mech mission.
When you do not get selected as mech pilot, you keep the licence, so you can use it for the next mission, and so on.
When you get selected, you loose the licence, but get one token for a special roll of the wheel of fortune.
So you don't loose anything for doing the mech missions (except when you get disconnected before boarding the mech).