It appears that you have these relationships:
Thus, if you place a portal to LIneItems on your Contacts layout, you should see all lineitems for items the contact has purchased. But they will not be obviously grouped by the invoice to which they are linked. If you were to add another Tutorial: What are Table Occurrences? of Invoices like this:
You could add fields from Invoices 2 to the rows of your portal to line items to include information from the related invoices record.
You could also set up a pair of "master-detail" portals where portal 1 lists all Invoices for the contact, and when you click a row of this portal, portal 2 lists only the lineitems for the invoice that you just selected in the first portal. See this thread for how to set that up if you are interested: Need layout solution for nested portals...
I've tried the LineItems portal but nothing shows up...
The"master detail" solution isn't what i'm looking for, since i don't want to click rows... if possible
If nothing shows up from LineItems, then there is something wrong with how you have set up your system. Either you do not have things set up correctly in Manage | Database | Relationships, do not have the correct values in a match field, or your layout is not set up based on Contacts with the portal referring to the correct Tutorial: What are Table Occurrences? of LineItems.
Never mind, I got it.
There was a little error on a portal field. Thank you.
Your explanations made me see things in another perspective :)