For some channel managers, for example Staah we can import the commision as part of the housing revenue automatically, check with Nitesoft support what is possible for you Channel Manager.
To add the comission as part of the total rate manually.
1. Go to the reservation where you want to add the comission.
2. Add a comission extra to the room and set the rate to the amount of the comission.
Read more here on how to create a new extra.
3. Lower the total price of the room to the price excluding comission.
Read more here on how to lower the price.
4. 'Save'.