Unless you have a supplemental income from a retirement or pension, don't even think about working as a substitute rural carrier especially in a small town. They pay you $18 an hour, sure, but then you only get 8 - 24 hours A MONTH.
This was apparently different for sub carriers in larger metro areas with more than two drivers.
Not only do they expect you to be on call all the time. They also expect you to service and maintain your own vehicle. With barely any pay to speak of to begin with.
If there was no work you could go to another town 30+ miles away and sub for them if the post masters spoke to each other in that region.
When you did sub for a driver, it was up to that driver to give you a map or directions on where to go. If you either lost or had issues with your directions you can and will get lost in the middle of nowhere.
You were also never efficient at sorting the mail for a route because you never fully memorized a particular area. By the time you did start to retain it, you were off to the next route.
You were also expected to finish a route regardless of time spent. If you delayed the mail you were threatened with automatic termination.
Driving was easy
Never had a chance to remember a route to be efficient