I know to get the last id I can use insertGetId() but I want to know how can I get the last inserted id through save() method.
order =
You can get the last inserted id by the following codes.
Using Laravel Eloquent
$order = new Store_order();
$order->invoice_id = $signed['invoice_id'];
$order->save();
$lastInsertedId = $order->so_id; //now getting Last inserted id
echo $lastInsertedId;
Provided that $order->so_id means last inserted id of the given object where so_id is the primary key of the table.