They’re known as Airmail Etiquettes.
We still get letters like this all the time from Africa. I hate opening them because they’re easy to tear.
I send letters and CD mailers overseas frequently and the folks at the post office told me letters/envelopes and smaller items are never sent by ship anymore. Everything is air mail.
I don’t know if there’s some size or weight above which it becomes a ‘package’ and subject to travel by ship.
Also, FWIW regarding changes to US postal regulations in the last year, I have had the same size/weight CD mailer priced out at 3 different reates - with the postal employees arguing with each other about it.
Wouldn’t it have been more polite to closely read the OP before posting?
Wouldn’t it have been more polite to point that out rather more gently? To err is human; your comment says far more about you than it does about me.
The question appears to be settled, but here’s my contribution. I work for a UPS Store, and we used to always use a red stamp that read “AIRMAIL” on international first class mail. Then when postal rates changed in about May of last year*, we were told to stop using our stamp. Now we just put proper postage on it (.90 to most countries, and I think it's .68 to Canada or Mexico) and send it on its way without incident.
- Incidentally, what are the official regulations as to Postcard vs. letter vs. large envelope/flat vs. package? We can’t get a straight answer from the post office. I don’t even think they know.
Do people still send much stuff by surface mail? I guess really heavy stuff still gets sent that way.
I remember (and this is not long ago, maybe 10 years ago) my dad would always send out Christmas cards to our two relatives in New Zealand in about September every year so they could go by surface mail and save about 20 pence. (This is a man who on weekend shopping trips would always spend 20 minutes driving round block after block of residential streets until he found a free on-street parking space, even if it was a mile from our destination, rather than pay 50p for a car park. Even if this meant spending three times that amount in petrol. But I love him…)
Edit: UncleFred, it appears that you can still send postcards and letters by surface mail from the UK at least (but only outside Europe for letters). See here.
46p for a postcard or letter up to 20g, as against 54p or 78p by airmail.
Wouldn’t it have been more polite to read the link before posting? Sure, he points out something that the OP knew, but the article is germane to the discussion.
Here’s a rule to live by. When you make a mistake, it’s not the fault of the person who points it out.
I think that should be made into a sticky in every forum. 
I think they say “Par Avion” because French was the working diplomatic language of the Universal Postal Union when it was founded in the late 1800s. Therefore, no matter what other language a post office worked in, it could still handle French.