Do not be surprised if real people actually die from this mistake, from the anxiety, the surprise, the helplessness.
Having that serious an underlying health problem means everyday life represents risk for you. I don't think that means everyone else has to behave differently wrt (in this case wrt to billing mistakes) to keep you healthy.
I'm stressed thinking about it, like from a theoretical standpoint.
a smaller error by say, just one or two orders of magnitude are much more believable as a reader
Just because you've not seen it or cannot fathom it happening in your world doesn't mean it doesn't happen.