Airlink de*****ing me out of R1610

Booked a flight and paid for excess luggage (20kg) to accommodate my rifle case. I paid R1610. I then called Airlink to notify them that I will be traveling with a rifle. Airlink told me that I should not have paid in advance, and that I would have to pay AGAIN when at the airport. They told me that the R1610 I have paid already cannot be used in lieu of paying at the airport. Neither can it be refunded. So, I would have to pay R1610 too much. The process when making a booking is very unclear and Airlink is very opportunistic and blase about treating their customers fairly.