Poor customer service

it is so useless and has poor customer service. we had load shedding in the mall when i went to purchase my son's milk. on my way back a realized that i bought the wrong size. wanted to buy the 1.8kg Lactogen baby formula and accidentally i bought 900g. i looked for the proof of purchase and couldnt find it, an hour or two later i went back and straight to cashier that assisted me, she did admit helping and i told them that i lost my proof of purchase, and i need to exchange it for the bigger tin and pay the difference. no one was willing to help me for an exchange, i was told to go look for the proof even after i had told them that its lost. they saw that the milk wasn't even open. they couldn't even be bothered, what kind of customer?