where the access unit at each layer except the lowest is,
A drone delivery drops in from the top to an empty locker, thereafter the locker is given an unique ID, a lock code and an access pin. The buyer receives the locker ID through an app on her smartphone. The buyer goes to the locker with her smartphone, locate the locker using its ID and keys in the lock code on display at the locker into the app. The app and its backend verifies that the buyer has an outstanding collection at that locker and that the locker ID and lock code are the right pair. The app sends the access pin received and the lock code to the backend server and opens the locker. The use of a lock code ensures that the buyer is at the locker. It is the access pin, hidden from the buyer, that opens the locker. The lock code and access pin cryptographic pair changes with each use, but it is the backend server that verifies that the smartphone app has an outstanding collection at that locker ID that provides security.
Good night...
