Worst online shopping customer experience ever

I have a complaint regarding an online order which I have placed in the time of purchase. The selected item was in stock and the transaction was processed and payment was taken successfully. A few days later I have received an email from mrp confirming that the items I have paid for is out of stock. I do not understand why the online app indicated stock was available and after my money has been taken, indicated that there is no stock available. I'm disappointed and it's not the first time this had happened. I gotten an email to confirm that my parcel is in stock to be collected and when arriving and checking my items. There were 3 items missing from my order. The amount for that items was never credited back to my account. This is a huge inconvenience due to the fact that I could have used the credit to purchase other items in store. This is so unprofessional and I don't think I will order online due to the recurring stock issues and failure to refund credit due for out of stock items. I have emailed Mr price no response what so ever and they still haven't refunded my credit back for the 3 items that wasn't delivered.