Because the Grand Canyon is so very remote, the people who live around the park and in surrounding towns have to wait to receive many goods and services. Especially in harsh weather, they're lucky to get mail at all, so hats off to the USPS for finding an affordable solution that facilitates tourism as well, because you can still mail your post cards direct from the Canyon floor. Although certainly it would be quicker and in someways easier to deliver mail by plane or helicopter, the Canyon isn't wide enough to take off and land, so the mail has to make the 7,000-foot descent some other way. It can't be done nearly as cheaply, either.
The people of that area still need their mail delivery. It's interesting, as it seems in this age of technology that there would be a quicker, easier way to get this mail delivered. I'm surprised that it still arrives via mule. At any rate, the postal service is designed to serve all Americans, so let it go on.
Yes, the post office at the bottom of the Grand Canyon should be kept. It serves a population of people who may not have access to postal service at other locations. They need a connection to the "outside world." To deny services at that location would be wrong, even if it were more cost effective.
The Post Office tells the story of America. The post office is the history that every person in America has a right to the same services that America offers. Every person can go to any post office and send a letter for the price of a stamp. These small offices are important because it brings equal rights to all.