They refuse to exchange items

I bought student prince school shots for my son at Pep in Woodstock. I would like to return them and take a bigger size as the one I got is small, the issue is that I lost the slip and the staff at their stores are refusing because I don’t have a slip. I contacted their office and was told that they don’t do exchange without slip. I am so disappointed and I think it is ridiculous that I am stuck with the pants just because I lost the slip. The shots still have the price tag and are in the store bag, surely that should count for something?