how to model this invoice requirements

In some business domain, I received invoicing requirements as follows:

+ Invoice Items are of two types: a service or a fee.
    - Invoice Service Item is taxable. fee is not.
+ Invoice Items (services and fees) cost is calculated in two manners: 
    - As a whole: the service or fee has a fixed cost.
    - By Individual: the cost is multiplied by Individuals count.

I’ve been reading about design patterns, domain driven design, inheritance and polymorphism, and I wanted to practice what I learnt in this side greenfield project. I want to come to a model for the Invoicing sub domain, that captures the domain ubiquitous language. I’ve come to this model, but It doesn’t feel right!