not the same thing, but in the supply dept at work i went to get batteries for office equipment. the supply clerk asked what was i doing, selling them. as long as she was the supply clerk i insisted she take the dead batteries back when i went for more.
you might just ask what the rules are so you won't be chased into the parking lot again. she may have been having a bad day too and might not have realized how she was behaving.
i hope it works out for you at the shop and that your graduate work goes well.