See if they are providing you plain bag then only we should pay for it at a resonable rate but if there is a logo of their firm then why we should pay for free advertising.
What if they are charging Rs 2 for a carry bag with advertisement and Rs 5 for without any ad... How can you fight then? It should be chargeable because they had to purchase those bag and print their logo on it. Otherwise they have to include the cost with the product. You can't asked for a discount on any item for not taking carry bag.