To create order rules, you need the Order Rules module, if you would like to purchase this, please raise a new case online and reference the title of this article.
The ChangeOrderValue Order Rule action lets you set the desired Order Value field on the order. For the Param field enter the value you want to set.
To create this order rule please use the following setup:
Action: ChangeOrderValue.
Param: Enter the Order Value you want to set.
Conditions: Set any required conditions.