how to fix an overpayment mistake

There's nothing wrong with what you did, bmetter.

It's just possible that the Automatically apply credits feature is activated. This is the reason why QBO is automatically marking the invoice as paid using the credit that you made.

Let me share the steps on how you can turn it off. Here's how:

1. Go to the Gear icon and select Account and settings.
2. Proceed to the Advanced tab and click Automation.
3. Look for Automatically apply credits, then slide the bar to the left to turn it off. 
4. Click Save and select Done. 

Once done, the credit will no longer apply automatically to future invoices. However, you can follow these steps on how to use it when the time comes that the customer wants to use it:

1. Click the + New button and select Receive payment.
2. Select the customer and proceed to the Outstanding Transactions section.
3. Look for the open invoice you want to apply the credit memo to.
4. In the Credits section, select the credit memos you want to apply.
5. For the open invoice in the Payment column, enter how much of the credit you want to apply.
6. Leave the Payment method, Reference no, Deposit to, and Amount received fields blank.
7. Fill out the rest of the form, including the Payment date.
8. Make sure the total is correct after applying the credit memo.
9. When you're done, select Save and close.

Additional details and guidance are also shared in this article: Create and Apply Credit Memos or Delayed Credits in QuickBooks Online.

Don't hesitate to let me know if the credit is no longer applied to the invoices automatically. You can also post more questions if you need anything else.