Appoling service

I feel disrespected and undermined by the staff at dischem. They had the audacity to tell me I must pay the difference of the merchandise I came to exchange. I had bought 2packets of nappies on the 25th of Aug and I realized now that I needed to get a different size. The lady that authorizes I suppose is the supervisor said I have to pay the difference as they are back to normal price. She bluntly refused and said I have to pay the difference. I don't know if this is in dischems exchange policy but it is the most ridiculous thing I've ever heard and I have never heard that before. Why should I pay for goods that I already purchased.