It's nice to have you here in the Community, @renewservices.
I'm here to guide you on where to add the sales tax on your invoices in QuickBooks Online.
To add the taxes, you can go to Products and Services, then assign tax categories to your inventory. Setting up your tax categories helps you calculate sales tax on invoices.
If you haven't set up your sales tax, you can click Add a sales tax on your invoices. Refer to this article for more information and detailed steps: Set up your Sales Tax.
Here's an article for your reference about assigning tax categories to the items and services: Add Sales Tax Categories to your Products and Services.
For future reference, you can refer to this article about how to design your invoices and different ways to send an invoice to your customer:
As always, you can leave a comment down below or start a new thread if you have any other concerns. I'll be keeping an eye on your response.