Khun-Khun Drugs Ltd review: Short change

I went and bought some food at Shopper's Drug Mart. I had over 8000 points on my Optimum card so I was eligible for a $10.00 reward. My bill came to 13.04. I gave a $20 dollar bill. My total minus the $10 should have been only $3.04. I wasn't paying attention as I go to this store quite often and trust the employees. I was still charged the full $13.04, my change should have been $16.95. My points were taken and I still paid the full price for that purchase. Now I lost the points and lost my trust.