Air India has never charged a convenience fee for booking a ticket online, the ticket price mentioned is what a person pays.
However many of the private airlines are charging a convenience fee which varies from rs 150 onwards per ticket
However as a promo in 2018 they are waiving the convenience fee for booking using the webapp. the promo is usually also available for booking using a laptop, though it is sometimes no explicitly mentioned.
So for online booking, it is advisable to check all the offers available.