We model the system consisting of three groups of agents interacting together: Senders of package, Receivers of package, and carriers.
Initially all of them are randomly spread geographically.
Then some Senders and Receivers randomly are willing to interact, and searching for the available carrier. All carriers have different radius of availability (suppose humans may walk maximum 6-10 km, car - 200-400 km, jet - over 10000). So the condition of deal is carrier containing receiver and sender within his radius of availability and who is free now.
Connections between inactive agents deteriorate with decay rate, after some time receiver and sender forget about each other and may form other connections in the system.
All agents have possibility to churn, and it is higher if they don't have any connections (any interest in the system)