Well i'm not what you can call an expert but what i see...
You are getting the serial number with the while statement
so of course you are gonna get as $serial the last record, so when you are deleting you will be deleting the last.
$qry_del="select * from order_detail_table";
You should pass the serial or id from the form and then just do the delete.
I hope it helps, i'm just a newbie
$qry_del="DELETE from order_detail_table where order_id=$order_id AND serial='$serial'";
$msg="item deleted success";
$msg="item not deleted";