I understand the importance of automating the discounts when the customers pay within the time-frame, xenaranda.
With the older updates of QuickBooks Online, we can set up the invoice terms to automatically apply discounts when you receive customer payments earlier than the due date.
Now that we have a separate Discount feature, discounts should now be applied on the invoices or sales receipts manually. You can refer to this link for more information: Add a discount to an invoice or sales receipt in QuickBooks Online.
I'll take note of your feedback regarding the importance of the feature. You can always visit the QuickBooks Blog and the Product Updates pages to see what's new within the program.
We'll be right here if you need anything else regarding the discounts and invoices.