There are still deals with USPS. I just shipped some telescope counter-weights in a medium flat rate box the other day. Max weight is 70 pounds. My package weighed 46 pounds. It probably would have cost triple the flat rate price to ship via common carrier.
I heard a story in the WSJ a few years ago about a builder that was doing a project on the North Slope of Alaska. Normal freight rate would have been $20,000 to get his materials there, so he MAILED them all in flat rate boxes (69 pounds each), and saved himself $17,000 in shipping costs. I don't know if the USPS has corrected the loop hole or not, but since it was an option, he took advantage of it.