Mr. Price home online shopping is unfair

I received very poor service from Mr. Price Home Parkview with an online order that I made. One of the items was not sent with my online order. This item is in stock in the stores First they told me at the store that I only paid for 6 of the items but I paid for 7. There was a discount on spending a certain amount When they do not have that item in stock online they refund the item taking away the discount. Why don't they source the item from the store? On purchasing the item at the store I had to pay in extra. I phoned the customer services and they told me they would send me the full refund for the item which never happened. They sent me a voucher minus the discount. Unsatisfied with this because I paid for the item which they did not have yet it is in their stores.