Can I turn off the option for customers to pay by credit card (make this the default) and only turn on when a customer asks to pay by credit card?

Thanks for reaching out to us about your invoice payment options in QuickBooks Online, @jbutler5.

I want to make sure you're able to get the information you need about disabling certain payment methods. Please know that when you enable QuickBooks Payments or online payments, the options for credit card is set by default. At the moment, there isn't a way to disable it, so you'll need to uncheck the option as you process the invoice.

You can also manually adjust the invoice by manually adding the  convenience fee. Just in case, I'm adding the steps here to add the convenience fee item:

1. Click the Gear icon, then choose Products and Services.
2. Press New.
3. Choose either Non-inventory or Service.
   
4. Enter a desired name of the fee.
5. Select an Income account, then click Save and close.

If you want to include the interest on the existing invoice, here's how:

1. Press Sales, then go to the Customers tab.
2. Choose the customer, then look for the past due invoice.
3. Add the convenience fee item in the unused line, then enter the amount.
4. Click Save and close.

Please feel free to come back to this thread if you have other concerns with QuickBooks. I'll be right here if you need further guidance.