In an integrated-services packet-switching network (e.g. the Internet of the future, which is expected to offer real-time as well as non-real-time services), the charging policy must be service-dependent, but how should charges vary with the type of service and with the quality of service (i.e. with the QoS parameters and with their respective values)? We start from a list of the properties the policy should have, and derive from it a formula that satisfies most of them. We also briefly discuss the evaluation of the coefficients in the formula and the experiments that could be run for its validation.