Abstract: A system extension for a billing system adapted to generate customer bills associated with the delivery of telecommunications products and services, wherein the billing system includes software instructions adapted to be executed on a computer server platform, a database for storing product configuration data for a plurality of product offerings and charging tariffs attached to the product offerings, and a user interface allowing a user to interact with and configure portions of the system, the system extension comprising: an editable derived attributes table that identifies a unit charge rate for tariffs attached to product offerings; and a charge calculation function for calculating a charge for a tariff attached to a particular product offering based on the unit charge rate identified in the derived attributes table for the tariff.