if you have just one shipping method you can create Shipping method in data.xml folder with( noupdate="1") option and then you can use it in your create function like this

data.xml file :

<record id="record_in_xml_id" model="delivery.carrier">

            <field name="name"></field>

            <field name="type"></field>

            all necessary fields 

        </record>

in python file 

shipping_id = self.env.ref('module_name.record_in_xml_id')

sale_order_obj.create({

            all necessary fields 

    'shipping_id' : shipping_id.id
})