We support creation of invoices or expenses with multiple items if the trigger side supports it. A popular source with valid multiline triggers is Freshbooks.
However, we do not support the creation of non-matching line items yet (creating extra line items without matching line items on the trigger side)
If you want to insert a price with taxes included, then using “Line item total amount” is your way to go, on the other hand if your price is taxes excluded using “Line item unit price” is the right choice. “Line item total amount” has preference over “Line item unit price” if both fields are filled.
If your invoice is taxes exempt, you can add an explicit 0 as tax rate.