I have a gelding who is the same way. We drive carefully with all pony-cargo & it does not matter if its the dead of winter or mid summer, he gets to his destination soaked. He also loads and un-loads well, does not care if he is alone or hauling with a friend.
I haven't thought to try a calming supplement for him but if you are looking for one pick up some Valerian root from your local health food store. Feed her a small handful of capsules an hour or two before you go, might be worth a try. It smells aweful so be prepared to make a paste and syringe the herb OR make a "soup" out of alfalfa cubes and feed that it will hide well in.