Still in private beta (which does not help) but isocket.com would provide ease in billing, reporting, and adjusting rates. Worth signing up for notifications a

Still in private beta (which does not help) but isocket.com would provide ease in billing, reporting, and adjusting rates. Worth signing up for notifications as a future solution :) Have never used this http://www.openx.org/ http://www.openx.org/ - but removes the rate adjustment problem since you are dealing with a marketplace. Might be worth investigating as well.