In this column, Cecil answers in regard to business mail
This leaves open the question of private mail with non-letter objects. Bricks are of course famous from the story of a building in a small town in Utahbuilt from bricks that were mailed because mail postage was cheaper than freight companies. (When the post company introduced a 50 pound per person per day limit, the city fathers enlisted the private citizens to mail many packages.) But there’s frequently the question if this would still work today. Well, in their mini-myths episode, the Mythbusters tested this with a coconut, and it worked.
It certainly makes sense that if someone applies a label to an object and pays the postage to ship that object that the post office would comply. As long as the object is not toxic, explosive, dangerous, etc. The trick seems to be in getting somebody else to pay for your object’s mailing.
That’s the problem with applying a reply postcard as a label. It isn’t intended to be a label, and the pre-embossed postage stamp is intended for a postcard return. So the post office updated their rules to apply sense to the situation.
Actually, it means taking unfair advantage of the (early) Post office not being savy enough to dress up their rules in legalese and close all loopholes to pranksters. Costs of moving mail have gone up in the decades, but the postages rate hasn’t been raised as high, partly because of automatisation (and partly by reducing the number of mail-carriers, raising their workload). So the post office thinking about a higher rate for non-machine-sortable letters, or a longer time for delivery, makes sense from a business standpoint. But because the post office also provides an essential service (and is in some way or other influenced by the government), they can’t operate like a real business.
In the example of mailing bricks because freight was too much shows this quite well: postage was lower, because under usual circumstances, packages were far less, so the mail service could deliver them in addition to normal letters. Freight companies had specialized in dealing with big bulky items, with wagons and horses, so they charged rates that made business sense. When the town people resorted to sending bricks through the mail, they brought the whole mail service for that area to a grinding stop, because scores of heavy packages piled up, which the lone mailman couldn’t deliver like normal packages. It cost the mail company much more to move the packages than they got in postage. Basically, the town made the mail service eat the difference to the real cost, as quoted by the freight companies.
Not every mail service in every country has this catch-all clause: they can just as well state that a package must be wrapped in paper and not exceed a certain size or weight in order to be transported.