Need some help understanding how to add an instance of an existing item to an invoice

I have a SSP script that is attempting to create an invoice for an existing customer with a fee determined by what they applied for.  I've set up the fee items under the noninventory items and I have a query which successfully gets the internal ID of the desired item.  Next the script attempts to create an invoice and in doing so I am attempting to add the item using the internal ID that I have found.   The problem is I can't get my head around how this relates, teh schema browser shows a field called itemlist which is of