Additional fields (lines)

Hello,
please give me advice
I would need to make “Volume product* of inventory (listed on the invoice)” as an automatically calculated additional field product in invoice

I’ve already tried several options, but I’m doing something wrong

I don’t use this. But I believe that in if you add a Complementary Attribute to the proposal/order/invoice then you can select a “Computed field”

Maybe this can be used to do what you do?