@Andrew461
Yes, one way is to go to her reservation - click on their photo in calendar and booking details will appear. At the bottom is Alter or Cancel. Click on Alter and then Change Reservation. In the subtotal section you can increase the amount you are charging. Then send this to the guest - they need to accept it for the revised price to be charged to their card. Note that they willl incur an increase booking fee however. So really this is for additional nights rather than 'add ons'
So...there is also a link to Request Money - I believe, but open to challenge on this, that using this option will mean no additional service charge is incurred by your guest.