Dont know about birth certificate as I was born in hospital hence my parents had all of the required documents.

But for the marriage certificate you and your spouce have to be physically present, along with an eye witness. I just did a year earlier. If you are planing to use it out of nepal it has to be translated. The office is in Babarmahal. you have to pay few rupees cant remember well how much.

Hope this helps

Sampada