Hello,

To solve this you need to create your sales order and then call the "set_delivery_line" method on this object. I have written the code as a scheduled action to find a specific delivery method and find a specific sales order (you need to change this to create). The code will then call the set_delivery_line on the sales order object with the delivery method and price of the delivery method.

Code:

# search for the delivery method in products
delivery_method = env["delivery.carrier"].search([("name", "=", "Free delivery charges")])

# get the created sales oder
sale_order = env["sale.order"].search([("name", "=", "S00003")])

# create the sales order line for the delivery method
sale_order.set_delivery_line(delivery_method, delivery_method.fixed_price)

I hope this helps. 

Thanks,