Can a class code be added to a sales invoice discount % that is calculated by QB?

Good to see you in the Community, mkleintx.

I'd be happy to provide some insight regarding the discount that you may find helpful.

Assigning a class to the Discount field is not possible in QuickBooks Online. However, you can create the discount as a Service type item and manually add it on the invoice so you can apply the appropriate class code. The steps below will guide you through the process:

Create a discount service item

1. Click the Gear icon, then choose Products and Services.
2. Hit the New button.
3. Select Service.
4. Name the item, for example, "Discount."
5. Enter all the necessary details.
6. From the Income account drop-down menu, choose the account for discounts.
7. Click Save and close.

How to add it to the invoice

1. Click the Plus (+) icon, then choose Invoice.
2. From the Select a customer drop-down menu, pick the correct customer.
3. Enter the Invoice and Due dates.
4. Fill in the necessary details in the Product/Service, Description, Qty, Rate, Amount, and Class fields. I've attached a screenshot below for your reference.
5. Click Save and close.

For additional insight about this, I recommend following this article: Discount as a line item on invoices and sales receipts

That should get you going in the right path. Keep in touch if you need more help with this, or if there's anything else I can do for you. I've always got your back.