Pnp packet milk

Good Afternoon <br> <br> I purchased 5 packets of 1 litre of milk from pick n pay on Tuesday evening I was in such a hurry I did not notice the date on the packaging , I got home and opened the one packet of milk as my 18month old son drinks fresh milk all day, not realising the milk was off, the expiry date was the 11 October, however I did not notice this until I saw my sons bottle the next morning, the milk was sour, I then opened about packet and noticed the same the saw thing.<br> <br> I then took the remaining packets of milk to the nearest pnp, which was in vangate mall on Wednesday evening but they refused to change the milk as I did not have a receipt for it.<br> <br> Now my question is why would I keep a receipt for 5 litres of milk? <br> <br> Utterly upset as to why milk dated the 11th would still be in the fridge on the 13th.<br> <br>