Hey, flatjaf. We don't offer first-party delivery but we do allow you to delegate work to "executor" organizations through our connections feature. For example, Meadow uses this feature to assign tasks to their partner dispensaries. Likewise, this can be used to work seamlessly with third-party executors like couriers.
The Postmates API is certainly an option for businesses.
Often, however, it's quite expensive. Our customers save a significant amount of money by executing deliveries first-party or by delegating to a partner organization.
Additionally, we strive to give businesses complete control of their branding and of the recipient experience even while delegating tasks to another organization. For example, we offer a quite customizable recipient tracking page and SMS and voice proxying to hide customer and driver phone numbers. There are more details about these features on the features page (https://onfleet.com/features) and sprinkled through our API docs but feel free to reach out if you have any questions: http://docs.onfleet.com/