Hello there, Adam. Allow me to chime into this thread and get this into detail.

In QuickBooks Online (QBO), you can create a service item for the discount to make it nontaxable then subtotal each line item amount for it to calculate each discount in the Product & Services.

To do this, you can turn on the feature first and create discounts on the Product and Services page wherein you can add them to your invoice by choosing the correct discount. Here's how:

1. Click the Gear icon, then Account and Settings.
2. Select the Sales tab on the menu.
3. In the Sales form content section, select the pencil icon to edit.
4. Choose the Discount checkbox to turn it on.
5. Click Save and then Done.

To create a discount for the items, please follow these steps:

1. Click the Gear icon, then Products and Services.
2. Click New to create a new item.
3. Select Non-inventory or Service for the type.
4. Name the item Discount.
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. From the Sales tax category drop-down menu, select Nontaxable.
8. Click Save and close.

In addition, here's an article you can browse through in case you need assistance recording invoice payments: Record invoice payments in QuickBooks Online.

Lastly, I've also added this article on how to personalize and add specific info to your sales forms: Customize invoices, estimates, and sales receipts in QuickBooks Online.

Let us know if you still have further queries about estimates and invoices in QBO. We're always here to lend a hand. Take care.