You can list all line items via the API and apply a null filter on the invoice foreign key to isolate items not tied to an invoice.
Endpoint: GET [https://api.eventtemple.com/v2/line_items](https://api.eventtemple.com/v2/line_items)
Example Request:
HTTP
GET /v2/line_items?filter[invoice_id][null]=true HTTP/1.1 Host: api.eventtemple.com Accept: application/vnd.api+json
For query details, check out the Line Items documentation on the Developer's Hub.