You can create a discount item, magnus1.

This will let you add the discount when creating the invoice with discounted items.

1. Click Gear icon and then Products and Services.
2. Select New to create a new item.
3. Select Non-inventory or Service for the type.
4. Name the item Discount. If you want to create multiple discount items, give them unique names so they're easy to tell apart.
5. In the Sales price/rate field, enter the discount as a negative amount.
6. From the Income account drop-down menu, select the Discount given account. QuickBooks creates this account for you when you turn on the discount feature.
7. Select Save and close.

When it's time to create the invoice, on the first row you can select the item, then the next row will be for the discount item. Note that this discount item is on a fixed rate. Create multiple discount items if you need more.

Let me know if there's anything I can help you with.