By all means use her name, because as you say that's what would be natural. You'll also need to find some way of showing that she's the bookkeeper, without the owner saying, "Get the credit card from carol, the bookkeeper"! That wouldn't work at all. But don't assume the reader remembers her name later - the name will have most likely passed out of their awareness as they know that Carol isn't important.
IN a very real way she's not a character at all; just a plot device. (Poor Carol, I'm feeling quite sorry for her now!), so don't invest anything in her, but do use her name.
Unless she does turn out important. In which case DO write the scene where the heroine goes to get the credit card and draw Carol reasonably well. But if you do that and don't bring her back it unsettles the reader. They've invested somewhat in Carol, and it's like a loose end hanging. We're always telling the readers what's important and what's not, (red herrings notwithstanding), and they trust us so we need to honour that trust.