Shocking service - selling a product they have no intention of supplying

I booked my daughter a flight from Cape Town to Johannesburg for the 8/9/18 and a return flight for the 16/9/18. Reference number LGQACA. I purchased her seats in the front on the isle for both flights as she gets sick when she flies and likes to sit in front on the isle. As Kulula offers the service of purchasing yours seats, at R130 per seat, I expect her to be seated where I have booked. The flight to JHB was fine, but her flight to Cape Town was delayed by 20 minutes at the boarding gate. When she got to the boarding gate, she was issued a new boarding pass and her seat had been moved to 15A - not the seat I booked and PAID for. When she queried this she was told that the aeroplane had been changed so the seats had been reallocated and there was nothing they could do about it. I think this is absolutely disgusting and unacceptable. If I have paid for a specific seat, I expect that seat. The aeroplane had a front row with six seats, I had booked and paid for 1D, and they gave it to some random person who did not pay for it. It is not like the aeroplane had a different configuration of seats. In my opinion this is equivalent to *****ing. You are selling a product that you are not providing, and the attitude of the airline is - sorry but that is the way it works. Kulula's service and staff is becoming worse and worse, and less and less helpful. It is unfortunate that they are affiliated with Discovery, as that is the only reason that we fly with them. They have a huge chunk of the market through this affiliation, and they are giving less and less attention to customer service and satisfaction. I am appalled at the level of interest that they have in assisting their passengers. I contacted the call centre this morning and was told to contact the customer relations department, but they only have an email address, no phone number. What a surprise, they know that they will be inundated with calls, so they only provide an email address as people are less likely to take the time to send an email. I expect a refund on the payment for the seat.