Ad serving firm Falk eSolutions is partnering with a European firm, Fivia, to create an integrated offering of both ad serving and agency media inventory and billing management, according to ClickZ. Integrating existing billing functions at large and mid-sized ad agencies has proven difficult, not least because the two or three major billing systems players have decided to drag their heels in integrating third party systems, such as many ad servers, that could one day supercede their functionality.