You got a lucky client for not paying a sales tax, daa1. I will walk you through the steps to add it to their invoice.
Your question is tagged to QuickBooks Desktop and QuickBooks Desktop for Mac, so I’ll provide the steps for both. I’ll start with QuickBooks Desktop first.
If you haven’t set up your sales tax, we'll do that first.
After the setup, delete the invoice payment because it’ll be applied differently if we’ll just add the tax to the invoice without deleting it. Once deleted, open the invoice and you’ll notice a TAX column on the table. Select Tax on the line item to indicate that it’s taxable, and so the invoice amount will be recalculated. Then, pay the invoice twice: 1 from your customer, and 1 from your end. Be sure to put a memo about this change.
If you're on QuickBooks Desktop for Mac, you'll follow the same process. Just check the User's Guide and go to page 186, the Setting up tax rates and agencies section.
You can run sales tax reports after editing the transaction.
If you have other questions about sales tax or anything about QuickBooks, let me know.