Getting overcharged for on sale items

Bought a heater which was on sale for R799. I bought 2 smoke detectors and batteries with it. I paid for it. When I received the heater there was no smoke detectors or batteries. I then checked my receipt and Takealot charged me full price for the heater which was R400 more. I contacted them and there answer was that they had to cancel my first order as and then when I finally paid 8 minutes after the order was placed they had to charge me full price as you you cannot purchase 2 sale items. I could not return the item as my sister already started using it. So for paying for my original item 8 minutes lates Takealot charged me R400 more while the item was still on sale. Ripp off and Tereible customer service. They need some competition as this is taking advantage of loyal customers.