For overall order total you can create a SKU, call it

Promo

or whatever, but in order to have a negative value, it has to be set as Other Charge (for resale if you buy from Vendor or sale if you cover the cost yourself etc). Inventory item cannot be used with negative amount. There is also possibility to use Discount Item More here : https://system.eu2.netsuite.com/app/help/helpcenter.nl?fid=section_N2248474.html (change data center in URL to your locale, mine is eu2)