Examples are half board, bicycle hire, a sightseeing trip, parking etc which are booked together with the accommodation.
Go to SETTINGS -> PROPERTIES BOOKING ENGINE -> UPSELL ITEMS. Select the property you want to set. If you have multiple properties each property can have its own rules so set up individually.
Select whether the item is optional or obligatory. Obligatory items will be automatically added to the booking and cannot be removed by the guest. An example would be an end of stay cleaning charge or a tax. Optional items can be selected by the guest with their booking.