Indigo Airlines — Unfair trade practices

I have an international flight and wanted to book additional 45 KGs of prepaid baggage from indigo. they allowed me to book an additional bag and 30 kgs in 1 go but now when I am trying to add another 15 kgs, they are not allowing me to do the same. customer care told me its there policy to allow only 30 kgs and charge for the remaining at airport rate. I did not see any such notice or policy details at the time of booking. this to me is an unfair trade practise,