The latest mail art I received from R.F. Côté came enclosed in an unused aerogram. Aerograms were typically sold by postal services with pre-printed, or franked postage. This Canadian one cost a mere 15¢ at the time it was sold, I assume back in the 1970s. Reg had to add some additional 21st Century postage to guarantee I would receive it.
The trick with aerograms is they were made of thin, airmail paper and had to be folded and sealed by the sender. No enclosures were permitted. Their light weight meant they cost about 30% less to send than an airmail letter.
In most countries they were purchased directly at the post office with the pre-printed postage. You could also find ones at office supply stores where you would then need to affix a postage stamp.
With an aerogram, once the writer had filled the page, they were finished. If you typed, you could cram a lot into that letter. Back in the 1980s they were an affordable way I and my college-aged friends would communicate. As email became popular in the 1990s the use of aerograms started to wane. The US Postal Service offered them until 2006, but I can’t imagine the sold many in that final decade.