Steve:

Are you looking for the invoice to be able to show the total square feet?

Item A 3 sq ft 5 units sold = 15 sq ft
Item B 2 sq ft 3 units sold = 6 sq st
Total sq ft this invoice = 21

Is the sq ft per unit for item A always 3 sq ft, etc?

If you added IC and used the weight field, would that get the job done?
Another option would be OE standalone, again using the weight field.