Is there a simple way to add Credit Card fee when paying for Invoices (either Partial or Full)? P.S. I don't want to apply any CC fee on Sales Order and just on Invoices payment.

No simple way for surcharges as far as I am aware, but I did build an extension for this previously for paying invoices in My Account. Took about 2 days. From memory I also had to setup a custom permission as the standard customer center role doesn't allow applying more to the invoice than what is due.

Did you build the extension for CC fee or Invoices payment (this feature is OOTB)?