You can set the portal to slide up/also resize enclosing part. This sliding will only be visible when you print/preview or PDF your layout.
Better yet is to print from a list view layout based on your portal's line items table with fields from the invoice record placed in the header and a trailing grand summary.
You can see an example of this in Comment's Invoice demo file:
I tried to do the list view version but I am struggling a lot with the layout. I tried to follow the system in Comment's invoice demo file, but the sub-summary areas werent displaying properly and when I print the invoice an individual sheet for each individual item comes instead of an overall list view.
Maybe the first part of your solution would be easier for me than the list view layout? What do I need to do to set the portal to slide up or resize enclosing part? I didn't see anything about that under the portal setup menu, can you give some more details?
Make sure your layout is set to list view, not form view.
If you add sub summary parts to your layout, you must also sort your records by the "sorted by" field specified for that sub summary part.
Object sliding is an option you set in the Inspector (FileMaker 11) or select from the format menu (older versions). But don't go there until you make at least one more try with your list layout.