Perform "Add to cart" through web services
I have found a similar post on stackexchange, which is little bit helpful.
Click here!

But there, they have used commerce line item's ids which will not available in my case because how could we know the lineitem's id before submitting it to cart ?.

Found the below solution for my above query.  But I am not sure, what are the standard way to implement this.

1) First I have to create an cart order using commerce services module.
http://localhost:8080/drupal7/serviceendpoint/cart

2) Second I have to create new line item and attach it to the order created above using the same module.
http://localhost:8080/drupal7/serviceendpoint/line-item

In that way I am able to create a new order with line items.  But I will appreciate if anyone could suggest me a clean, better and a proper way.