You can purchase or buy Instamojo Credits from the Appstore. These credits can be used to subscribe to any of the Apps in the Appstore. You can use any of the available online payment options to buy the credits.
Credits once bought, stay in your account forever till they are utilized. The purchase of credits cannot be canceled or refunded. Similarly if you have subscribed to an app, you can cancel the subscription any time but the credits deducted from your account cannot be credited back.