When Transactions like Sales Order, Invoice are integrated via REST Web services, the expected data structure needed for adding transaction lines is an object with an array of line item objects.
Here’s an expected input to sync or integrate line items of an Invoice/SO along with the body fields.
{ “items”: [ { “item”: { “id”: “<NS item internal id>” }, “amount”: 149.36, “quantity”: 1844, “rate”: 0.081}, { “item”: { “id”: <NS item internal id> }, “amount”: 5.55, “quantity”: 37, “rate”: 0.15} ] }