Thefumegator,
I'm going to advise you not to carry. Now, that doesn't mean that I'm advising you to stay at that job. You can, of course, move on to something that doesn't make you feel threatened by the people you meet. But as it is, you're working there of your own free will, and you're going to have to abide by the rules and laws of the place that you choose to be at. Do I agree with the rule? Of course not. Not in the least. But we make Faustian deals in our lives, and must abide by them at times to see that we don't muddle up our lives in the future. The chance of your getting "made" while carrying (in a manner that could be helpful to you in an emergency) are higher than the chance of you getting shot at in your (armored, right?) toll booth.
Me? I'd probably move on from the toll booth gig, but I don't know what your circumstances are.
In the mean time, just have a contengency plan of what you'll actually do if the balloon goes up. If someone points a gun at you, see that the window's closed, drop, and get out of the line of fire. Even if they could stand up and shoot down, that's a much higher angle shot, which makes in much less likely to penetrate and hit you. Also, that would necessitate their exiting their vehicle, which puts them more at risk. How good's the lock on the door? Could a supplemental dead bolt or throw bolt be in order? Do you keep a phone where it can be reached from the floor? (Not just cell phone-- you want to be able to dial 911 and get a response to your location.) Is the area below the counter hardened? Could you keep some armor there? Do you have a panic button that can be reached from under the counter?
Considering these questions and producing solid answers may well serve you better than having a little gun stashed away in deep concealment.