Fascinating, but can't happen in Australia.
Postal regulations ... anything on the reverse of an envelope/postcard is not valid for postage.
Interesting...lots of countries have no problem with stamps on the reverse; indeed, it some countries (India, China come to mind offhand) it was customary
to put the stamps on the back, and only the address on the front. Leading to a lot of 'opened out for display' covers in albums.
Canada Post has had 'flip over' labels for quite a few years. The tracking labels for things like Xpresspost (express service) are rather large, so if you're mailing just a No.10 envelope, once you put the address and stamps on the front, there often won't be space for the label, so they stick it on reverse. The label used to be simpler, just plain text, they've 'upgraded' by adding the illustration, which is a useful aid for international mail where people might not be able to read the instructions (semi-related, the Victorian-era "Leaf" definitives were criticised terribly in Quebec, for having only English
inscriptions, and hence were re-issued a year or so later as the "Numeral" issue).