Overcharge and don't care

Just less than a month ago, I ordered 2 x 6l of long life milk, amongst other groceries. The products were delivered. Just that instead of 2 x 6l, I got 2 x 1l cartons and was charged R64.99 for this. I went through the order with the driver, was told to make a note of the error on the invoice, I did so, and also informed the customer care of the error via email. A week later, I had to send a follow up email due to no response. I then got a response full of questions about whether I checked my order and whether I made note of the error. I rep**** and waited another week of no response and had to send another follow up email. After that I got an email saying that they will follow up with the store about the refund, and it has now been another week since then! Why must there be so many hoops to jump through just for approx R40?? It was an error on Pick n Pays side and I cannot just leave it and let it go, regardless of it being a relatively small amount but at this point I am just tired of sending follow up emails and waiting for weeks and I don't think this is acceptable when it was their error.