None of the 2 links you provided explain this and I already check the articles in the search you just sent and I don’t see one explaining how to do this.
For working with Line Items, in most cases, both the sending and receiving app will need to support line items. For more info on line items and how to use them within Zapier, check out our help article here: https://zapier.com/help/create/basics/use-line-items-in-zaps
You will need one set of line items for each field in the invoice that requires line item data. If all line items will have the same value for a specific field, you can hand type that value and we will assign that value to the field for all items added.
Once you have the required set of line items for each field required by the invoice, you can assign those items to the fields in the Quickbooks step that are contained inside the Line Items section.