You can set up the extra services you offer by going to:
Account Settings > Services & Charges > Other Charges & Discounts. These services will then appear in a drop down menu under "Add Additional Charges" in the Cost section of the booking and you can select one as appropriate.
Alternatively, you can add ad-hoc services under "Add Additional Charges" by selecting Other from the dropdown menu and completing each field box.
If you offer discounts you can use negative numbers in this section and the system will deduct these charges from the balance.