@Helen3, thanks for your responses. After getting in touch with Airbnb, they reached out to the guest and got the payment issue cleared up quite quickly, so that is settled.
I had no intention of letting them stay without paying, but the question was whether the issue would be resolved and what the potential cancellation situation would be, since often times these issues, which should be clear-cut turn out to be rather ambiguous for us hosts. I'm satisfied that that part of the booking has been resolved.
Now, for the first issue, I think we hosts do indeed have to be concerned about asking guests, particularly first time guests, who often do not understand quite how Airbnb works for simple things that should not be a problem.
I have never gotten a negative review, let alone one for an issue, but I've read dozens of stories on these threads about some guests indeed leaving poor, retaliatory reviews for being asked reasonably to pay extra or to follow the rules. I think it's best to be proactive and seek out others' experiences and considering all options before addressing an issue that one has never had to deal with. Myself, I have tried to prevent these issues from arising in the first place by being very clear with my rules and policies and communicating them with potential guests clearly and efficiently, but there is always, so it seems, going to be that one guest who falls through the cracks.
Asking this woman to pay the extra money might indeed upset her because let's say she didn't read or comprehend that part of the rules/messages or because she's cheap or ignorant? It could happen!
Thank you again for your input and Happy New Year!
John