We are doing this now.  However, we add a markup.  When the invoice is created and sent to the client it groups the markup amount in with the original category.  We want the markup to show up as a separate line on the invoice.  So lets say we have supplies and a markup, we want the invoice to show 2 lines, one for the supplies amount and one for the markup.  That way when we get the invoice in the other company we can separate the markup from the supplies amount.