I think much of it has to do with insurance and liability. Initially, when I was put on Bridge City Tools' waiting list for a Jointmaker, I was told that USPS would not insure it for delivery, whereas UPS and FedEx would. So, I was prepared to swallow the cost and go the insured route when the order was filled. Two months later when I got lucky and they came up with a Jointmaker for me, I was told that USPS would insure it. So, I had it shipped USPS happy to avoid UPS and FedEx handling and brokerage charges which would have been substantial on an order of that high value.
I got the order problem free with no duty or handling charges, but I think now that I was given the wrong information when I was told that USPS would insure an order like that. In any event, it all turned out well, so I'm complacent.
I'm guessing that order costs and details are more quantifiable with the bigger shippers.