How do I correct/change an invoice (remove a charge) that has ALREADY BEEN SENT to a customer?

bring the invoice up on screen, find the item 

right click and select delete

or

change the wording to show it was already invoiced, and set the rate to zero (I prefer this method since it leaves an audit trail

Hi there, Majordave.

I'd like to share what I know about customer transactions.

QuickBooks lets you track and edit invoices. We also can manage the payment that was applied to the invoices to make necessary changes.

If you'll edit or change the amount, we can open the invoice and apply some changes.

Here's how:

1. Go to Sales on menu ☰.
2. Click on Invoices.
3. Locate the Invoice.
4. Click the drop-down arrow under Action.
5. Select View/Edit.
6. Make the necessary changes.
7. Click Save and close.

Also, you can try deleting the payment and reapplying it as a partial income after you modified the sales transactions.

Like this:

1. Open the invoice.
2. Click the payment link below the Payment Status.
3. Click the payment date to open the Receive Payment window.
4. At the bottom, click More then select Delete.
5. Press Yes to confirm the action.
    

Once done, go back to the invoice and edit the items then click on Save.

Here's an article that will help you process customer payments: Record invoice payments in QuickBooks Online

Let me know if you have follow-up questions or clarifications. I'll be in touch. Keep safe!