Hi there, @KrisG9988.
Thanks for posting here in the Community. I'm here to help share some instructions about adding the credit card processing fee on your invoice.
To add the credit card processing fee on your invoice, you'll just need to add another line on the transactions field so you can enter the details and amount.
Let me walk you through the steps:
I've attached some screenshots below for your reference:
Also, I've included here some helpful articles that you can check out about customizing sales forms in QuickBooks Online for your future use.
That should do it!.
Please let me know if you have any other questions with adding credit card processing fee. I'll be around to keep helping. Have a great weekend!