I've come to share information about your discount concern, @agolius.

In QuickBooks Online (QBO), the total tax amount is calculated based on the total value of the invoice's items. With that said, when you manually add a discount value after you've selected all the items in the form, the taxable amount will change. 

You can set the discount as a service item to prevent changing the taxable amount. This way, the discount will be manually calculated based on percentage. I'll show you how:

1. In your QBO account, go to the Gear icon.
2. Next, choose Products and Services.
3. From there, click the New button.
4. Select Service for Products/Services type.
5. Then, in the Name field, type Discount. If in case you want to create multiple discount percentages, you can enter the percentage and Discount for the name. For instance: 10% Discount.
6. Next, from the Sales price/rate field, input the discount amount as a negative.
7. Then, select Discounts given account in the Income account drop-down.
8. In the Sales tax category section, it depends on your state if sales tax is calculated after discounts are applied then mark the discount line taxable.
9. Once done, click Save and close.

When finished, you can now add the discount item to your invoices. For your reference, please see this article: Add a discount to an invoice or sales receipt in QuickBooks Online. 

Additionally, feel free to visit this link to further guide you in adding sales tax to your invoices: Applying Tax to transactions and sales forms.

Just in case you want to have a list of your customers’ outstanding balances to determine how much is still due and how long they’re past due are. You just simply need to run the accounts receivable ageing report in QBO. 

You can always leave a reply below if you have any other concerns in applying discounts to your invoice transactions. I got your back. Have a good one.