Refused to Exchange product

I am really disappointed at the service of the checkers in the new Malvern Park Shopping Centre. I purchased 5 boxes of Pronutro for my son for the month. Unfortunately, I took the banana flavour instead of original. After he opened it and tasted it, we realised our mistake. I tried to take the remaining 4 sealed boxes back to exchange for the original flavour, and they refused to assist me as I no longer had my original receipt. I can show that I purchased it from them through a bank statement and they no me as a regular, but they still refused to assist. I will no longer be shopping at this Checkers.