Immunity to Mental Stun

Immunity to Mental Stun is alluded to on page 55 of GURPS Zombies under the notes for Unfazeable.
It mentions that Mental Stun is classed as a Common threat.
B81 has a base cost of 15 points for common which is the final price from the x1 immunity multiplier.
This is described as:
:immune to not only Fright Checks, but also “freezing” due to surprise (p. B393).
This implies that someone Unfazeable could normally still be stunned by surprise (since it is not based on fear)
Someone with Immunity to Mental stun without unfazeable could still presumably be frightened but would not be mentally stunned if that was the result rolled on the fright check table.
Despite this overlap there appears to be no cost reduction for having both advantages. Although you could get a 1/5 cost reduction for making them Alternative Advantages, those need to be switched via a Ready maneuver, which would mean you couldn't just use whichever was convenient on the fly.
category:advantages