Hello, Evan. 

I appreciate sharing your insights and solutions regarding the sales tax calculation problem.

I'd like to address the Tax box issue when creating an invoice in QuickBooks Online. 

Just to clarify, QuickBooks Online does mark the line items as taxable automatically provided that the customer and the items are taxable. 

You mentioned that you use Harvest for invoices. Did you integrate it with QuickBooks Online? If so, check Harvest's app settings and see if there any settings that causes this. 

If you use QuickBooks Online and Harvest separately, this might just be a simple webpage lag issue. You can refresh the invoice creation screen page to fix this. 

If it still persist, you'll want to check to see if this is a cache-related problem. Isolate this by using an incognito window: 

• Google Chrome: Press CTRL + Shift + N.
• Safari: Press Command + Shift + N.
• Mozilla Firefox: Press CTRL + Shift + P.

After opening the incognito window, log in to QuickBooks and create a test invoice and check the Tax box. If it checks the box automatically, go back to the regular browser and clear the cache. 

When the prompts are done, close out any browser instances to finish the process. You can also use another supported browser since each browser uses a different cache data.

If you'd like to check on your financial data after fixing the Tax box issue, you can read this article to see what reports you can run: Run reports in QuickBooks Online.

Do you have any other concerns related to sales taxes? Or do you need help with other things such as expense transactions? Let me know the details below. I'll help you out.