That's unfortunate of course but there's nothing you really can do. You should have been a 100% before publishing the listing.
It seems unfair to the guest to have to pay for something that wasn't listed in the beginning but at the same time its unfair to you but you can't blame the guest for your mistake although you have my full sympathy.
You could ask for them to pay or ask to cancel as you have a flexible cancellation policy. But you might risk being perceived as petty and risk a bad review as the guest might feel uncomfortable ( which i def would if somebody put me in this position).
Or you could accept the mistake and raise the the price just slightly to compensate for this loss.
If I were you, I'd let it go and considder it a leason learned.
All the best