Related Questions:
Can I get a list of menu items from the API that aren't associated with an invoice?
How do I use the API filter to show me menu items without an invoice?
Answer:
Using the Event Temple API you can List All Line Items and then filter the response to only show Menu Items that are not associated with an Invoice.
Here is an example of a request using a filter to sort out items without Invoice Id's:
https://api.eventtemple.com/v2/line_items?filter[invoice_id][null]=true