Let me share some information on how you can add the credit card processing fee to your invoices in QuickBooks Online (QBO).
To add a credit card processing fee to your invoice, you can create a service item, and then manually add it to your invoice as a line item.
Here's how:
On the left panel, go to Sales.
Select Products & services.
Click on New then choose Service.
4. From there, enter the name of the service item.
5. Select the account you'd use to track your processing fees.
6. Once done, click Save and Close.
You can now then add the credit card fee as an additional item on your invoice when you charge your customer with the processing fee.
On the other hand, you can also look for a third-party connector app that can be integrated into the program to help add the processing fees. To find one that can sync into your QuickBooks account, you can follow the following steps:
Sign in to your QuickBooks Online Company.
On the left panel, look for the Apps.
Choose the Find apps tab.
Enter the app's name in the search bar or select the Browse category dropdown menu for you to check out the app.
Click the Search icon.
Lastly, I've added these articles in case you need assistance personalizing and recording invoice payments in QBO: