This article explains how to apply a mass discount to multiple items on an invoice using the mass discount function. This discount is applied as a percentage (%) and can be made permanent for the client. Remember, mass discounts work only on unfinalized invoices.
Note that you can use this function if you have the required permissions.
On the invoice page, select Tools > Mass discount at the bottom of the invoice.
Select the Item types that you want to apply the discount for.
Add the discount Percentage. If you want this discount to be permanent for the client, select the Permanent checkbox. You can view, edit, and delete the discounts you have set as permanent on the client's page, on the Billing tab under Client discounts.
Select Save.
See also how to set up a client discount or create a discount scheme.