@Charging upfront and refunding if they do t use the heater as expected @Charging upfront and refunding afterwards if they do not use the heater every day.@Kelly-and-Natalia0
You make it very clear in your listing that there is a free, how much and what it covers.
then as part of your email exchange you agree with the guest the fee and that they wish to avail of the amenity.
once the booking is confirmed you go to the Reservation and click on Change/Cancel. You amend the cost to include the total accommodation fee, cleaning fee plus the pool heating fee. You do not add the Airbnb service charge as this is done automatically. Your guest will then need to accept the new price. If they don't then you will only be paid the original amount.
an alternative would be to bill the guest afterwards for days they used the heating butbthis may lead to disputes and difficulties. I would suggest Charles