It's my pleasure! I'm excited to hear that QuickBooks is lining up as a match. I can absolutely assure you that the out of scope option is a default once sales tax is configured in QuickBooks Online. You're right about it being needed in order for the line-item discount workaround to work properly.
Here's a screenshot from a live trial to show you what it should look like.
I just set up sales tax on that account and didn't have to do anything extra to get it to appear. Just like that, you'll be on your way.
I just realized that in my last response, I forgot to link you to our videos and tutorials page! Hopefully this is coming better late than never, but here's our support page that shows you how to find help articles as well as video tutorials: How can we help? This may have been the page you were looking at originally. If not, I can see what else I can track down for you if you give me a few more clues. Let me know!