how to get it:-
Create/Login to your FreeCharge account
Select bill payments/recharges, and choose your operator
Transact for Rs.400 or more, and in the next step use promo code
50POST
Pay through preferred payment method
Enjoy the cashback in your FreeCharge Wallet
t&cs:-

Valid on successful First transaction of Postpaid Bill Payment done till
23rd March 2016
Valid on min. transaction of Rs.400. Flat cashback of Rs.50
50POST
Promocode has to be applied to avail cashback offer
Valid only once per User/Credit/Debit Card/Mobile Number
Valid on Credit/Debit card and FreeCharge Balance
Cashback will be credited to your FreeCharge Wallet which can be redeemed within 185 days from date of credit, and is non transferable
Not valid on Virtual Cards & International Cards which are issued outside India
Not valid for Netbanking & Airtel Postpaid/Prepaid/DTH/Landline
We reserve the right to end any or all offers at our discretion without any prior notice
http://offers.freecharge.com/50POST/