Where services are offered on a commercial basis, the manner in which charges for service usage are calculated is of key importance. Services typically have associated with them a charging scheme specifying the rules for charge calculation; schemes can range from the simple (such fixed charge per service invocation) to highly complex (where charges are calculated dynamically in order to influence customer demand and thereby optimise overall system performance). Typically, charging schemes are manually configured and verified prior to services being made available to customers - typically, a time consuming and expensive process. In environments where service compositions can be rapidly built and offered to customers, manual specification of a charging scheme for the service composition becomes untenable. In this paper we describe how charge modification rules associated with individual services can be used to flexibly govern how a service is charged for when it is used in the context of a composed service.