Hi Juli, you can explain the additional/optional charges in your property listing description and state it is not included in the listed price.
Then if the guest chooses to accept pool heating you can send them a "special offer" with the extra price added or if they have booked already with you, go into your reservations and "alter" the booking. The guest then needs to accept it. Alternatively (i guess especailly over the swimming season) you can can make the daily price inclusive of this already. I am not aware of a customers ability to select additional extras during the booking process. Hope that helps.