Has anyone applied the credit card processing fees to their customer's invoices? If so, how?

You can apply credit card processing fees to the customers' invoice by manually adding a service item, Shannon.

First, we'll have to create a service item. Here's how:

1. Click the Gear icon in the upper right corner.
2. Under the Lists section, select Products and Services.
3. Click New and choose Service. Name your service (e.g., "Credit Card Fee").
4. Select the account you want to use to track processing fees.
5. Click Save and close when you're finished.

Once done, open your invoice and add the created service item as an additional line on your invoice. Take a look at my screenshot.

For customers paying via ACH bank transfer, you have the option to let them pay the convenience fee automatically.
 

If you have more questions, feel free to reply below.