Hello @jamie-larochelle

Let's run the QuickBooks Utility tools and fix the incorrect calculation of the sales tax when creating your customer's invoice. Let me show you how.

A. To Rebuild Data

1. Go to File.
2. Click Utilities.
3. Select Rebuild Data.
4. Click OK.

B. To Verify Data

1. Go to File.
2. Click Utilities.
3. Select Verify Data.
4. Click OK.

In addition, here's an article you can read to learn more about the QuickBooks tools: How to fix data damage on your QuickBooks Desktop company file?

Lastly, I've got you these helpful articles you can read for ideas about how you can manage your customers and sales transactions: Sales and Customers - QuickBooks Desktop for Mac.

If there's anything else that I can help you with, let me know in the comments. I'll be happy to help.