Bad customer service experience

Afternoon, I bought pampers on special from Spar Parkrand on the 25th of August. I noticed that the size 5 I bought is becoming tight so I went in today to exchange for a bigger size 6. I was then told for me to exchange for a bigger size I must pay in 5hw difference between the sale and the normal price. I left the store without the exchange as it doesn't make sense to me to pay in if I am sumply exchanging for a bigger size. I have sone countless exchanges from many other retailers where the purchase price and price at date of exchange was different ans never had any issues with exchanging for a different size. How does this then make sense at Spar? Please assist with way forward. I have already logged complaints through Spar's online channels kn the 3rd of September and although they promise a 3 working day turnaround time, my complaint has been ignored even after follow-up.