Sabarmati Gas — Extra amount added to bill

I have a connection of Sabarmati gas.l have already paid the new connection amount ₹5000. But in each bill extra amount as Monthly installment ₹500 is added to bill which I already paid at the time of new connection. Ditails are given as :

Customer ID:[protected]
Meter No. [protected]
1. Invoice no: [protected], monthly installment paid ₹500.
2. Invoice no:[protected], monthly installment paid ₹500.
3. Invoice no:[protected], monthly installment paid ₹500.
4. Invoice no: [protected], monthly installment paid ₹750.

Please make corrections in my bill I shall thankful to you.

Thanks & Regards