In 1980 I was working in the camera department of a Canadian department store (Sears Canada). We sold lots of Kodachrome. As it was in Canada, each roll included processing by Kodak.
Working back into the mists of my memories, I think a 36 exposure roll of Kodachrome 64 was a couple of bucks more than $10.00 ($11.99 plus 7% sales tax?) and a 24 exposure roll was just under $10.00.
If you dropped your film off at a Kodak dealer, it would generally be returned to that dealer within 2 days, processed and mounted, without charge. A very small number of dealers were then making noises about charging a small handling charge, but most weren't
Otherwise, you would mail the film in to Kodak in the handy pre-addressed envelope.
The fact that processing was pre-paid was inscribed on the cannister itself - the mailing envelope had no monetary value.
It was more expensive to shoot negative film and have it processed and printed - thus a fair number of people would shoot slides when they were traveling.