Each Tariff is composed by a list of Fees. Tariff contains minimal logic and provides unified interface for calculating Fees. Tariff implements Fascade pattern to operate list of Fee objects.

• Tariff is a class representing the main entity the model is going to compare. Tariff, product, service and subscription are hereby considered the same thing. Tariff aggregates a set of fees. Tariff classes are designed to be reused when same pricing model is required, only with different values.
• Fee is is a class that comprises a Tariff - a Tariff has many Fees. Each Fee implement it's own 'Calculate(...). E.g. some fees are based on consumption, while others on fixed periods of time.
• Applying a fee is the process of calculation of cost - really done in IFee.Calculate(...). When Calculate(...) is called on Tariff object it calls Calculate(...) on each Fee and aggregates the result

To add a new Tariff extend the abstract Tariff class.

Extend and implement from IFee interface. Use the Fee within a Tariff class.

Override Calculate(...) from the base abstract Tariff class.

To add features to the model, like add more unit types or change calculation algorithm, re-implement the ITariff and IFee interfaces. Consumption class is also meant to be extended as needed