Fever adjust prices at a whim during a transaction

I bought 2 tickets for a certain price in a certain seating block. 2 days later I bought another 2 tickets in the same block. The computer showed the same price. I did not notice that the credit card payment auth was now R320 more expensive for 2 tickets. I wrote to them. They declined a refund for the overpayment. Nothing I could do as they had the credit card details.