Indane Gas — collecting more than bill amount

Address: Salem, Tamil Nadu

sir the gas lenders are collecting Rs.30 more than the billed amount as service charge . I think we are subjected to pay the billed amount and we can give tips if we are willing .The billed amount is 389.96 and i am paying 420/. do i have the right to oppose this .Are they paid for distributing the cylinders or we are supposed to pay the delivery charge .I just want to know my right.