I ended up calling Airbnb to resolve this one. They opened it back up so that we could send a second request. Then the guest accepted it, said she was going to pay, and didn't. I called Airbnb again and they had me use the "involve airbnb" button and they contacted her to collect the payment. They say she has paid, but we still haven't gotten the payment. So, basically, all a guest has to do is accept a charge and they can just not pay it, from what I can see. Now she is sending us angry messages. If she continues that, I will flag the messages for Airbnb to get involved again. We were not so concerned about collecting $5 from her as her deceitful behavior saying she is paying and not paying, and showing up with an extra guest and saying she "forgot" to include her in the booking. The only reason for this small extra charge in this particular room of ours is because we have to set up and take down an airbed when people want separate beds. She had one terrible review prior to us, which we didn't see until after she had instant booked. But she also had a good review after the terrible review. This is also a problem with the instant book feature, in my opinion, because we don't see the negative ratings until after they instant book, and, even though we are technically not penalized for rejecting instant bookings, they would still be able to review us since the reservation is already "in progress" at that point.
Thanks for your help.
Ruth and Neal