Worst online shopping experience

I ordered some groceries for my grandmother, who is unable to walk. Only half of the order was delivered, specifically the half consisting of low-value items like chips and sweets. The high-value foods, such as cereals, never made it. They called to say the rest weren't available in stock when the order was packaged, despite the website indicating they were packaged and delivered. Given that these are readily available food items in any grocery store, I highly doubt that explanation. I was promised a refund, yet there's been no action indicating the intention to send me back my money. It's been three weeks since the delivery.