There is no portal within a portal. You can solve this a few ways. Use a repeating field and make a fake portal. You can use SQL or something similar to do this. Or you can create a portal on a slider and get the primary id and put it into a global and show the second portal related to this. I did this with a slider so that is looks like a popover.
So when you press the button it opens the slider and set the primary key of the customer and then show the invoices in this second portal