Does the Shopify API support adding new line_items (products) to existing orders?
It's currently not possible to update line items for an existing order using the API.
The following documentation shows what you are able to update - https://help.shopify.com/en/api/reference/orders/order#update
So basically, when the order is created initially is the only time you can assign it line items?
What is the business logic behind this?
Now you can