Refuse to exchange an unopened box of Long John

Went to a cape Union Mart store in Pietermaritzburg to purchase k-way thermalator long johns for my mother she told me to specifically ask for a Large. Got to the store and asked for Assistance because I had no idea what I was meant to buy. Told the first person who offered to assist me what I was looking for he then gave it to me,I paid n left. To my surprise when I got home he had given me the wrong size. My mother then asked me to return and explain what happened because she had already bought 3 others long johns of the same size before and they did not fit( she did not take them back ). Went back to the store the following day explain to the manager what happened and he simply refused to exchange the unopened Pack. Called customer service to get assistance, explained the situation and the lady just rudely answered by saying “I should just give them away because the store will not change the policy just for me.” Im honestly disappointing in the Service I received. It surprises me that a retailer of this size does no operate competitively with similar businesses, by offering the same services.

Would really appreciate feedback on how to go about this matter.