There’s a pretty large combination of factors that went on from what I remember. Partly it just plain didn’t get that hot that often in the UK or France back then. Partly they wore entirely different undergarments that layered to keep sweat off their actual clothes and keep them cool. Partly the summer clothes were often flowy or puffy which helped move air near the skin. And partly the fabrics they wore were different. Things like linen and cotton were the go tos. Even the linen itself was different, modern linen has shorter fibers and is much lighter.
After the great renunciation (when men started wearing the modern(ish) suit) you start to see a lot more references to taking off layers either to cool off or to keep them clean or whatever. You also start to see variations in really hot places like the American deep south and Bermuda to deal with climates that get way way hotter than Europe with its nearby seas and cooling prevailing winds.
Please note that all of this may be wrong and I am entirely going off what I can remember off the top of my head.
I’ve used stickermule a number of times, and signed up for their newsletter. They sent out a very annoying political sms blast and email like a week after they started their sms deal. They got a ton of bad press for it so I kind of doubt they do it again. Their normal weekly marketing email is usually just a sentence or two that says what’s on sale that week so it stood out. They have occasionally have the $9 for 50 deals that you can use on the same account, it seems like once every year or so. They were more common but they probably lose money on them as they have said a couple times they weren’t going to do it again.
The actual service is really good. They aren’t crap stickers, they use vinyl with good adhesive, their print quality is good, and the top vinyl layer is one of the thicker ones around. They misprinted a batch of mine and reprinted them the same day that I contacted them at no cost.
The only tip I have is to keep a design that you want printed ready to go. They cancelled a $9 for 50 order I had open but hadn’t uploaded after about 10 days. Which is kind of understandable but annoying since that’s not how their warning email says it works but whatever.