Extra baggage cheating

We were trying to buy extra baggage online. The price is R320 if u buy online 2hrs prior to departure or it’s 420 if u buy at airport. Kulula website didn’t allow us to book online for the whole night. It was showing an error. When I tried explaining this to the lady at the counter, she didn’t want to listen to what I say. She was insisting me to pay for the extra baggage at extra price. She didn’t want to check if there was an error. She said u have to pay anyways so just pay. This should be a way kulula wants to make extra money which is pathetic. They should stop this nonsense make make the price 420 fixed than cheating passengers with error.