The option to choose the Sales Tax Discount isn't currently offered in QuickBooks Online. In the meantime, you can choose Other as the Reason to specify the discounts.
I can pass along all the feedback you have to the appropriate channels so you're able to reach a point of resolution as quickly as possible.
Additionally, I'd encourage you to visit our QuickBooks Blog, so you'll be updated with our latest news and updates including product improvements. Just go to https://quickbooks.intuit.com/blog/.
These write-ups are good references for using the automated sales tax in QuickBooks:
If there's anything else I can help you with, please let me know. I'd be happy to help.