[1] The postal service (snail mail) physically delivers letters using a best-effort delivery approach.
The service will make their "best effort" to try to deliver a message, but the delivery may be delayed if too many letters suddenly arrive at a postal office or triage center.
The sender is generally not informed when a letter has been delivered successfully, unless one pays for this premium service.
The applications built on top of those protocols implement the additional services they require on an end-to-end basis.
Both transport-layers are the base protocols and provide multiplexing between processes on the same host running over different port numbers.