It's nice to see you here in the Community, copies of client 1099. Allow me to provide information about how sales tax is calculated in QuickBooks.
The option to toggle whether sales tax should be applied before or after discounts is unavailable in QuickBooks Online. This is because QuickBooks Online automatically calculates sales tax by default after a taxable discount. You can click the See the math button on the invoice page to ensure that the sales tax is calculated correctly. You'll be provided a detailed breakdown of the sales tax.
For more information on managing your sales tax in QuickBooks, you can check out this article: Set up your sales tax.
You may also want to override sales tax calculations in QuickBooks. Here's a guide on how to do it: Override sales tax on transactions.
Come back to this post if you have any other concerns about sales tax in QuickBooks, copies of client 1099. The Community will always be here for you.