Hi @Gisela0
I am not sure why you would even ask the guest for a cancellation fee. You are likely to get more booking with a "flexible" cancellation policy chosen but with it comes the risk of people cancelling and not having to pay a fee to the host. If you want to be compensated for people cancelling, I would strongly recommend you change your policy to "Moderate" or "Strict". Even if you had "moderate" chosen, you would not get anything from this gust since the booking is a week away.
In answer to your question, "How do I go about collecting this fee"?, you don't. I would chalk it up as a lesson learned and move on. I really don't know how you would collect it through Airbnb since your cancellation is set to "Flexible" and the guest did nothing wrong and she is already paying her cancellation fee (booking fee) to Airbnb. Maybe you can work something out that she wires you money or go through Paypal but it likely won't happen through Airbnb.
Cheers
Dave
David
Superhost Ambassador ~ Host Club Community Leader ~ Experienced Co-Host