Relationship between Item Fulfilment and Invoices

I am trying to create a gross margin report which will add the Cost of Sales field to each invoice item transaction. The cost of sales comes from the Item Fulfilment type of transaction which has a Created From field linking to the sales order. The invoices are also Created From the same sales order transaction. The problem is when there are multiple shipments or multiple invoices linked to one sales order,the query returns wrong joins. Is there another field that help match an item fulfilment to a specific invoice, other than the "Created From" field? Or, is there another