Bad customer experiencing for changing an item on an order

I bought an non returnable item about an ago but selected the wrong size among the 6 items that was in the basket. I phoned the call centre now asking them to just change the size to a bigger one which cost more and after being speaking to the supervisor it is clear that no one in the call centre have the ability to assist and that was repeated over and over from any question that I ask.

Bottom line is that if you make a mistake it is on you and they will not assist in rectifying your mistake confirmed by the supervisor named Grant. Clearly customer service is of highest priority before they have your money after that you on your own.