I'm fairly new to FMP, actually only 4 days into it but I signed up for lynda.com and have been watching tutorials on it. There was actually one that talked exactly what you are speaking about.
I think this will take you to the course but you'll be interested in chapter 5.
I know this isn't exactly an answer but maybe it will help you out.
You need to set up a portal to Line_Items on your Invoice layout, with fields from Products, selectable via drop down or popup value lists
Thanks Aaron and Symbister, this seems to be exactly what i was looking for.