You can set up the header as you described, then the body with one row of the order line item fields. If you set the layout (layout based on the orders table) to list view it will show how ever many line items there are, and go into multple pages if need be.
The Invoices Starter solution that comes with FileMaker uses the method Mark describes. You can take a look at it for an example of how this might be set up. You can also examine the same technique in this much simpler demo file created by Comment and uploaded to a different FileMaker Forum: http://fmforums.com/forum/showpost.php?post/309136/
It worked, thanks :)