Vacuum forming if I'm remembering correctly. Sorta like those space saver bags you see on TV all the time.
I remember in high school we used to have vending machines with an off brand soda in them (can't remember the name), but they'd put t-shirts into some of the bottles, along with a buck in quarters. It sucked because if you were really thirsty, you had to fork up more money for another one, or find someone with a knife so you could get the quarters out. Nice thing was the sodas only cost .75, so you got an extra quarter out of the deal.