Typical mistake. Service price is never based on the costs of development but on a value it creates. You might have extremely expensive IT solution that creates no value to anybody and is worth $0. You might have email-based manually operated business that provides service worth thousands USD.
Postmates creates value both for you and the delivery guy. After all - you used their service for some reason and paid for it right? You get the quality service, delivery guy gets the demand for his services.
Alternatively the delivery guy could develop an app for himself only, put it on the appstore and wait for you to download it. But he would never earn anything this way.