Should I add credit card processing fees to my customers' invoice? if so, how?

Hello userpurchase, 

Welcome back to the Community. QuickBooks is a versatile program that helps you streamline your work using the powerful features offered in the program. Rest assured, I'll be happy to share more info so you're on the right track. 

The function you've described isn't available in QuickBooks. I can see the benefit of having this function and I encourage you to send feedback about this to our engineers. You can send feedback by clicking on the Gear icon and looking for Feedback. New product ideas and improvements are based on the feedback we get from users like you. I also recommend looking into third-party apps on our app store using this link. In the meantime, I recommend manually adding the fee to the transaction and it's best to consult your accountant for expert advice on how to do this.

Otherwise, let me know if you have other questions by leaving a comment below. I'll be one message away. 

@userpurchase 

You will need to use a third party merchant service to integrate with QB and let your clients pay the fees.