Logistics are all about minimal handling. There is an often quoted line in the ocean transport business that it is cheaper to load a ship at Long Beach, sail it around the whole world and off load it again than it is to load it, sail it a mile out to sea, turn it around, come back in and and off load it, then do the same again.
A system that will get a huge volume of letters from extremely widely distributed points, and deliver each letter to its own individual address is a logistician’s nightmare. You need a massive standing system to accomplish it, with a vast asset inventory and payroll which leads to an enormous fixed cost that must be borne across the individual items. Every single item, no matter how light or small, requires individual routing and delivery. Volumes to any given address go up and down but the system must be standing by to work all the time.
By comparison, bananas are grown in certain reasonably fixed places, they are a uniform commodity that can be shipped in bulk to a very limited number of addresses, and volumes are known and steady.
Think about this: if I gave you a truck with a container on the back with 20 tonnes of palletised bananas in it for delivery to 20 supermarkets (one tonne per supermarket) in your city how long would it take you? Two days maybe? If I gave you a truck with a container on the back with 20 tonnes of letters in it each weighing an ounce (that’s over 700,000 letters), all for delivery to individual addresses in your city how long would it take you? At a thousand a day it would take you two years, and you couldn’t work that fast.
I don’t mean to suggest that the postal system is as efficent as it could be or that prices are as low as they should be. But frankly the OP and your post is the type of ignorance that cause people in logistics to want to strangle someone.
This would only make sense in the context of oversupply. Where demand is steady the import rates will be matched (approximately) to demand so that limited shelf life will be taken into account. No way would the importers import so much that they are constantly putting themselves under pressure in the way you suggest.
As you say, dry bulk rates are irrelevant. However, general container rates are also irrelevant. The collapse in the container sector is due to lack of imports of consumer goods basically from China. A specialist trade like bananas would be very different. No doubt rates will have dropped a little but not much. I also had the feeling that bananas to the US were a vertically integrated trade ie the big banana companies own their own vessels, but a quick google hasn’t given me the answers I was looking for. But if I’m right charter/freight indexes have nothing to do with it.