Extra Baggage Rip Off

After booking a "cheap" holiday for the family, I originally only added 1 checked bag (due to it being a short holiday). This cost $20 each way during the booking process (after making the mistake of adding a Jetstar Club membership). I then realized after the booking, that I should have added another checked bag as apparently babies don't even get a 7kg carry on bag. I go to manage my booking to add another bag for my wife only to find that it is double the price now ($40 each way) for the same size bag. There is literally no extra effort on Jetstar's part for me to add a bag and pay for it during or after a booking is made! I then contact Jetstar customer service to be told that I should have booked the bag originally and that it is their policy. The guy tells me that it clearly states during the booking process that the cost will be much higher if bags are added afterwards (online or at the airport). While chatting to him, I went through the booking process again (in case I missed this clearly stated popup), only to find that it just mentions high cost adding bags at the airport (nothing about online edits). I sent him a screenshot to prove what I was seeing. He didn't seem to care and said that I was doing something wrong or their website was incorrect (which I have to pay for their mis-information). All he said was that's their policy and I must accept that! Charging people double to add a checked bag is crazy.