For the experiment of Exercise 3, name some variables the driver did or should have controlled. Was

Question:

For the experiment of Exercise 3, name some variables the driver did or should have controlled. Was the experiment randomized and replicated?
Exercise 3
A pizza delivery driver, always trying to increase tips, runs an experiment on his next 40 deliveries. He flips a coin to decide whether or not to call a customer from his mobile phone when he is five minutes away, hoping this slight bump in customer service will lead to a slight bump in tips. After 40 deliveries, he will compare the average tip percentage between the customers he called and those he did not. What are the experimental units and how did he randomize treatments?