Your system should not use a calculation field to show the price. What will happen to your older invoice totals if you need to change a price in the products table?
This is a case were you need to add a number field to your line items table that uses an auto-enter feature (looked up value or calculation) to copy over the price. Coincidentally, this added field will be editable--allowing you to override a looked up price.
Perfect, makes much more sense.
Thanks again Phil.