What happened to treating customers with dignity

I purchased a sweater from Zara on the 24/03/2015 as a gift for a elderly lady. Unfortunately it was too small so I went to the store on the 30/03 to exchange it for a bigger size. Unfortunately I lost the slip. When I informed the cashier, Phumza about this she informed me that she will not be able to exchange it due to me not having the slip.This was rather strange to me,as this was the first time in my history that a store did not want to exchange an item in the absence of a slip. She actually told me how would she know the item is mine. This was an indication to me that she thought I stole it, I have never been humiliated like this in my life. I then asked to see her superior, Londiwe, who came with the story that it is the company policy not to exchange without a slip. I was not satisfied with her answer, at this time my husband joined the conversation. He then asked for the most senior person in the store to assist us.Cheslin, the store manager,repeated the same story. At this point I asked them why was I not informed about this policy at point of sale. The store manager then informed us that all cashiers are properly trained and they should inform customers about this policy