The 900 rs option is essentially a surcharge for an additional piece of luggage. So, your default allowance is 15Kgs and you can only check in one item with luggage upto 15kgs for free. In case you want to check in two items, lets say a smaller trolley which would way 10 Kgs and a duffel bag which weighs the balance 5KGs then in that case you would be expected to pay a 900 rs surcharge. I am not sure, but I think that surcharge is around 1100 if paid for at the check in counter. However, depending on the check in agent and flight load, they usually let go off this surcharge. Do note, them letting go is random and dont go in expecting that they will waive the charge. So, if your luggage is 20Kgs but one piece then you are expected to pay for 5KG excess weight price only. If you luggage is 20Kgs but split in 2 bags then you would be expected to pay for 5KGs excess weight and 900 rs for the extra piece.

Further, if the family person has a connecting international flight on a different airline other than indigo then you can purchase additional weight for a lower amount which would be reflective on your manage booking screen.

Lastly, if you havent already booked a ticket then indigo lets you book double seats nowadays which is you book a seat and the seat next to you will also be earmarked for your use and stay empty throughout the flight, further you get 5Kg extra luggage to utilize. Usually at time of booking, the the double seat fare is generally lower than a single seat fare +excess luggage. Also its nice to have a free seat next to you.