I had noticed about the export certificate which allows me to carry goods without paying the duty. But there is a problem, I couldn't know where to get that and do we have pay for that?
You dont have to pay for it. Note: This certificate is to be taken when you're leaving India so it helps you on future trips into India.
It's available in the airport (or whichever international port of departure from India)
Usually - after the Immigration counters and before you proceed for Security checking, there are a couple of counters where White-uniformed Customs officials are seated.
You have to show them the goods - they might even ask for an original bill. You have to fill up a form, which states the price etc, and they stamp and certify it. This is the Export Certificate
Remember to carry it on your next arrival to India alongwith that item.