It's great to see you here in QuickBooks Community, @Roxror.

Yes, you can still undo the sales tax code in your invoice. All you have to do is to open the transaction again and change it from there. Let me show you how:

1. Go to the Sales menu and select Invoices.
2. Look for the transaction, then click it to open the Invoice window. 
3. Click Edit invoice and update the sales tax rate.
4. Select Save.

Please take note that you will be prompted once the invoice is already paid. Changing it will also affect your customer's balance. 

I'd recommend reaching to your accountant for further guidance before taking this action. However, if the transaction isn't yet paid, you can always reopen it and apply your changes.

For more details, you can check out this article on how QuickBooks Online automatically tracks your sales tax using the Tax feature.

Additionally, there are several financial reports you can run to track your sales income in QuickBooks. They will show you helpful insights on the things you buy and sell, and the status of your inventory. 

Let me know if you need further assistance with this. I'm always around to provide answers and clarifications to your questions. Have a great day ahead and take care.