Is this software totally free or need to pay after some time or any limit of transaction ? any limit of time ?

No 100% free no limits. We charge you for apps for premium services. You can see a list of them in the App Store to your left. All apps gets 30 days free and you can cancel at any time.
But other then that the full multi currency accounting app with invoicing, bank rec etc is 100% free no caps. That includes this instant support.