extras addiotional item price not added total amount

Hi, i have set up my booking so you can book an apartment for max 4 people. I have also set under extras that the third and fourth person pay 10 and 20 euros per night. when i select the number of persons in the for it says the additional price next to the number but does not add it to the subtotal amount. Is there any way to fix that?

https://wordpress.org/plugins/booking-system/