Value Field Update for Zero value

Hi

I have a specific requirement in COPA..We don't want to update quantities for Billing documents that are having '0' revenue and '0' cost. I am not sure whether it is possible or not?

We are using same invoice type for all billing documents. Otherthan invoice type is there any other options available to contol postings to COPA from a billing document?

Appreciate your thoughts.

Thanks

RK