Adding additional fee

I am very close to having a fully functional site for our family’s small B&B that uses this Bellevue theme http://bellevuetheme.com/ and the WP booking plug-in with woocommerce. I am very happy with it and I think it looks great. The one last piece of the puzzle is that I need to devise a way to charge customers an additional fee for more than 2 guests (our room rates are based on double occupancy and we charge an additional $25 for each adult after that).

Ideally, the customer would select their dates in the booking calendar and the number of people in their party in a dropdown menu on the first form and based on the number an extra guest fee would be added to their cart.

I am looking for any/all help or creative ideas on how I might make this last part of our site function. I want to make it as intuitive as possible for our guests.

THANKS