1. I'm sure you know this, but never leave her in the car if it's even a little bit warm- and even for a couple minutes- cars heat up fast, very fast. And depending on where you live, dognapping is a concern, especially as demand for dogs increases during this pandemic and dog theft rates go up.
2. Practice going as far as/leaving for as long as she's comfortable for and going back to reward her before she freaks out. You can gradually increase the time you leave her for.
If she does bark as you're walking away, freeze in place, wait for her to stop barking, then go back and reward her. Next time, don't go as far