The list and structure of a Button URL with all possible fields, including those for Line Items that are drawn from Child Records, utilized is below. Please note that many of these fields will not work unless the corresponding field is setup in the accounting system.
/apex/bread-winner__NewInvoicePage ?originating_record={!salesforce_record_ID} &parent_account=api_field_name__c &invoice_date=invoice_date_api_field__c &due_date=invoice_date_api_field__c &branding_theme=branding_api_field__c &tax_treatment=tax_treatment_field__c &invoice_reference=reference_api_field__c &child_relationship_name=child_name__r &child_object_inventory_code=inventory__c &child_object_description=desc_field__c &child_object_quantity=quantity_field__c &child_object_unit_price=unit_price_field__c &child_object_discount=discount_field__c &child_object_revenue_account=rev_field__c &child_object_tax_rate=tax_rate_field__c &child_object_tracking_code_1=code1field__c &child_object_tracking_code_2=code2field__c &child_object_sort_by=sort_by_field__c &testing=true
