Showing Invoices history of a customer on a New Case creation form

I would like to understand the relation between these entities: Account(Customer)-->Order-->Invoices. I already showed customer's order history in a sub grid on the Account's quickview form, which is then used on the new case creation form. Now I would like to know what sort of relationships do Account-->Order-->Invoices hold? So that I could show Invoice history as well on the new case creation form.