Hello there, @dananorthrup-mai. I want to ensure you'll be able to add a credit card fee to your customer's invoices in QuickBooks Online (QBO).

To do so, you can create a service item for credit card fees and add it to your customer invoices. Here's how:

1. In your QBO account, select the Gear icon and click Product and Services.
2. Click the New option and choose Service. 
3. Enter the name in the Name field (ex. Credit card fee).
4. Next, check the I sell this product/service to my customers section and select the expense account for the fee in the Income account dropdown.
5. Once done, click Save and close.

After that, create an invoice and add the service item as another line item on the form. 

When finished, you can use this resource as a guide in recording customer payments in QBO: Record invoice payments in QuickBooks Online.

I'm always here ready to help if you have any other concerns managing your customer's invoice. Please don't hesitate to leave a reply below. Have a good one and stay safe.