Do you have this relationship?
Contacts::ContactID = INvoices::ContactID
If so, you can either specify a sort order for the date field in INvoices in this relationship that sorts the records in descending order by date or you can put a one row portal to Invoices on your layout and specify the same sort order as part of portal setup.
If you use the first option, you can just put fields from Invoices on your contacts layout and they'll display data from the most recent invoice. With the second option, you locate the same fields inside the sorted portal.