You don't need (Actually don't want) two different tables.
Just add another ProductType field.

This is useful too if Services are not taxable and Parts are for instance.
When entering the items into the detail, you could also create a combo box to limit or filter down items.

For instance, say you are a mechanic.

Item                   Type
Spark plugs          Part
Air filter                Part
Hose clamp          Part
Rotate Tires         Service
Detail interior       Service
Tune-up               Service

Hey Scott, that could work, but I need to see the Invoiced items in different select. The 'Products' in one and the 'Services' in another on my Invoice Reports. Where each Selection Total is displayed and Calculated 'Complete Invoiced Total at the bottom.  What's the best way to make this work?

Check out the Grouping video.  
Sorry to be short but up against some deadlines today.  I'll try and bop in later.