Sour milk, sour client

On Christmas day we bought some milk at Pick n Pay in Rayton when we visited family in the area. When we got home on the same day, we discovered that the milk was sour. We live approx. 100KM from Rayton. So how on earth can we take this back? We tried phoning the customer care line, which was cloed (obviously, since it was Christmas). I then sent a message via their website's contact form. We are yet to hear from them. Today, 2nd Jan2018, I tried phoning the customer care line again, after more than 10 minutes of holding (I phone from a cellphone, so it cost alot) I hanged up. What am I to do?