Wait till 24-48 hours before departure. At that time you will be able to check in on AirAsia's website and select ancilaries. They dont display those bookings not made directly before check in, but do at the check in time.

OTAs sell ancilaries at much much higher prices that the airline directly, it is their main source of revenue afterall.

For BKK-KUL, a 25kg bag purchased around 24 hours before at the start of this month was 130 ringit or 30USD, which I think is quiet reasonable. You do have to buy it at least 4 hours before, as you can only do it at the airport after that time and at a much higher price. Also, they have the fast pass for around 15USD, which lets you take 14kg of baggage (and get fast track immigration at KLIA).

PS: Kiwi e ticket receipt should display the PNR that can be entered on airline';s website. Just checked one of my old tickets.