Relationship Invoice

Hello,

I would like to create an invoice.
My data would come from a "Customer" table and a "commission rate" subtable (customer table (soustable) and products (relation)) and an "invoice" table including the amount of each product ordered I would like to apply when I print my bill the commission rate of the sub table linked to the customer table for each product ordered.
When I build my bill I can not link the rate and the product.
Can you help me please