As a Canadian who has been in the US close to 10 years, this is the first I've heard of this! I had always assumed the flag was meant to be set by the letter carrier in some bygone era as a courtesy to let residents know they had mail. Thank you, kind internet stranger.

To answer the question, this service doesn't exist in any other country I've lived in -- Canada or Japan, in my case.

In the UK, there's normally a postbox serving a whole street that people can put letters in to send (distinct from the letterbox in each house's front door). GP's nearest postbox might have been inside their local post office, or they might just have needed to buy stamps.

I've never heard of it outside the US.

In Australia, you post something by putting it in a post box, they're located in shopping areas and have specified collection times, usually daily on weekdays.

Of course, you can also go into the post office and deal with a human who will take the postage $ as needed.

These days, the number of times I've posted something can be counted in low single digits per year. Mostly dealing with government forms that need signatures because stupid reasons.

Most houses don't have an external mailbox, the mail goes through the front door and into a basket. A lot of houses on terraced streets have no front garden and open directly onto the road, especially in cities.

There is doorstep letter and parcel collection for less than a dollar.

https://send.royalmail.com/

Does not exist in Germany either. Most of our regular mail delivery is done by foot or bicycle, except in very sparse rural areas. And our mailboxes have locks anyway. So it's either the post office or a street mail drop-off box/pillar box. Nobody I know does regularly post letters anyway, it's more in the range of 1-2 letters a year if even that.