Airtel — regarding charging rs.15 as late fee

Hi,

Dth recharges are done as prepaid.
On prepaid recharge, why do airtel charge rs.15 as late fees?

My account got stopped due to insufficient balance on 12 am and i recharged the same account by 10 am today.
But still they have recovered rs.15 from my account.
And these amount recovered from our balance is also not being notified to the customer.
So, i would like to seek information from this regarding this concern of late fees