Linking costs to amenities
Do you have rentals with and without sauna and do you want the guest to pay a surcharge for a cottage with sauna? Then you can link a cost item to the sauna facility. Costs will be calculated for all reservations for rentals to which this facility is linked.
Set selectability to mandatory and set the correct price calculation.
Under validity, specify that the "cost item is only valid for reservations with a specific amenity?" Select yes.
Then select the amenities that apply, leaving the checkmarks valid for "choose specific property" and "valid when choosing amenities" unchecked.
Want to calculate preferential charges when the guest chooses a specific property or amenity? Then read here how to set this up.