Hello usertandv7!
I understand that you want your customer to pay the processing fee. I'm here to help you.
There are different policies with regard to passing the processing fee to the customer depending on the State. Some of them don't allow such practice. This is the reason why this option isn't available in QuickBooks Payments.
As a workaround, you'll want to add it manually to the invoice. Make sure to be transparent with your clients what are these fees. You can check the detailed steps in this article: Add service fees manually to invoices.
Also, please note that the processing fee in QuickBooks Payments is being calculated based on the total amount.
Here's a link that discusses how to manage your funds in QuickBooks Payments: QuickBooks Payments FAQ.
I'm here to help you if you have additional questions. Take care!