Airtel — airtel dth late payment fee charged for pre paid service

Address:

641035

I am having airtel dth connection ([protected]). Iwas done my recharge for the connection on 15/may/2018 with 290 rs+ balance 4.24. But in my account i can see the balance only 279 rs.
What is the reason behind the money deduction?
Dth as prepaid why i want to pay 15rs and they have stopped the service when its expired. When i contact the customer care i am not getting the proper respose. Reference no is [protected]. I want to clear this issue and expect this never happend on next time.