Is there any hook which we can use to run some php code post an order status has changed e.g. Shipped to Delivered.
We want that the code should run on once the status of order has changed.
Sure, take a look at the 'fn_change_order_status() function in fn.cart.php and use the 'change_order_status' hook.
Note that this is probably the most common hook used in cs-cart so be wary and cautious of the data you encounter or insert.