How can clients pay for Online processing fees?

Hello there, @oscortega.

At the moment, charging your clients for an online processing fee when making a purchase is unavailable in QuickBooks. 

What you can do instead is to manually apply the charge to the transaction. 

First off, you'll need to set up Non-Inventory or Service item for the credit card fee. I'll show you how:

1. Click on the Gear icon located on the top right.
2. Select Products and Services under the All Lists column.
3. On the Products and Services list page, click ‘New’ in the upper right corner, then choose Service or Non-Inventory.
4. Fill in the necessary information.
5. Click Save and close.

When you create an invoice, simply add the Convenience Fee item. Check out this article for more insights: Add product and service items to QuickBooks Online.

It's also a good practice to let your customers know about the charge before you invoice them. You can enter a message in your sales forms. Read through this article to learn more about this process: Customize invoices, estimates, and sales receipts in QuickBooks Online.

Let me know if you need anything else. I'm always around to get back and help some more. Thanks!