Deducted amount for Carry bag

Whether it's Legal or illegal, You can't sue the seller for 7 Rs.
Standardized stores and malls include the carry bag amount in bill, non organised sellers don't have any rules and policies to follow.. e.g. some coldrink shops will charge you 2Rs above MRP for chillled soft drink.. and you can't do anything about it...

While asked. they r ready to provide new bag. can i file a case against it

Are you asking about bulk order for business or just a casual purchase for own use?