The option to pay using Balance will be shown to you automatically on the payment page if you have any credits in your account. You can then choose to pay using Balance or use other payment options as you wish.
You can check your account balance in the account section once you log in.
In case of an unsuccessful recharge where your payment has already gone through, we park your funds in the balance. It's 100% safe and available for you to use anytime at any of our merchant partners (including FreeCharge and Snapdeal)