Sphurthy gas agency Indane gas - Charging above the invoice on LPG

The gas agency or delivery boys are charging **rs above the invoice when ever we book a cylinder on demand and some delivery boys threaten to take back the cylinder asking us to collect the cylinder at their agency. We have an argument everytime we book a cylinder as this time invoice shows Rs*** and he charged Rs***.. they don't even bother to reply back and run by giving the change and start the auto, its an ongoing issue. We tried to call the supervisor number ********** who says don't give the money and hungs up on face.. else he will take our details and do nothing. So its waste of talking to supervisor nor the agency..