Missing Food Items

Before 08:00 this morning I was at the Drive-Thru McDonalds, Rustenburg CBD, besides the service being slow, when I got to work I found that there was an item missing in my order. I paid full price for an incomplete order. Can the staff be so *********** so early in the day???? I called the establishment and they said they'll get back to me. I want a full refund for the inconvenience. So in the meantime, while I'm waiting for their call, what am I supposed to do? Not eat until they call???? My receipt no was 1898.