By entering a negative value, the system will automatically subtract that amount from the booking total.
You can set up your discounts under:
Account Settings> Services & Costs > Other Charges & Discounts
Choose a discount from the drop down menu in the cost section of the booking under "Add Additional Costs".
Alternatively, you can apply adhoc discounts by selecting 'Other' in the dropdown box and completing each field box.
To apply a percentage discount to the total bill, add a negative amount in the QTY box (-10) and change "Fixed" to %.