Bad shopping experience

I have recently bought a school pants at PEP and the size is wrong, and have lost a slip. The pants still have tags but they say they don't exchange without a slip and so far I think it's a only store that doesn't exchange. I have proof from the bank statement that I did make a purchase but still they won't assist. For me it's a very bad experience I don't wanna even shop at PEP anymore coz if it happens that you loose a slip with your purchase it means you have just throw away your money