Breach of contract

I purchased a two ticket from Jhb to CT and back. Because I am tall, I paid an additional amount of money to reserve a seat at the emergency exit. Additionally, I confirmed my seat by checking in online prior to my flight. However, when I booked in on the day the seat (THAT I HAD ALLREADY PAID FOR!!!) was assigned to another passenger on the very morning. The person tried to convince me that it was a system error. If their system is so unreliable with making a simple booking (FOR A SERVICE THAT THEY OFFER) what more serious issues are at play as well? Long story short, I had to accept a different seat because the other passenger couldn't be moved - EVEN THOUGH I WAS THE ONE WHO PAID FOR THE SPECIFIC SEAT. Now I must spend even more time to send them an e-mail to claim my money back. Kulula is not only wasting my time and money, they are wasting my life. A very dissatisfied customer!!