The Postbus can be found in two different projects in the Orchestration layer. The first one is Handlers.Default.BusGamma and the second one is the Handlers.Default.Plugins.
The BusGamma is specific to NserviceBus implementation. This handles the request put in the bus in the PreBus layer. Separation of BusGamma handlers and execution of actual business code into Plugins help to keep the bus code decoupled and helps to change or swap the bus whenever needed.
In the PreBus the command is put in the bus after processing the validation and the handler further processes that from the queue. This helps in separation of concern and all the actual business logic resides in the plugins..