Each Portfolio has a subscription associated with it, and there is always one user who has been assigned the responsibility for paying the subscription fees associated with the Portfolio; the Payment Contact. If you have received this notification email, then you have been nominated as the Payment Contact.
You may receive this after having already supplied your billing details. In this case, Observant are simply updating the Portfolio's payment information and there is no need to make further action.
For further information see Observant Global Subscription Fees or contact the HelpDesk.