I am buying it from ebay but 91mobiles provides coupun code. It shows in the site like this
How to avail cashback:
Go to eBay and purchase a product using coupon code 91MOBILE09. Ensure that you see "Rs 0.01" discount on your product.
Complete your order & go to Claim Your Gift Voucher - 91mobiles
Submit your email ID, eBay PaisaPay ID (Order ID) & bank a/c details.
Cash back will be processed by 91mobiles in 40-45 days.