The bottom has a cutout hole shaped like a Guitar Pick and this is where the picks pop out after being cut. Some of the Guitar Plectrum Punch’s on the market come with a few sheets of thin plastic that you can use to make many picks with but all of the same width as far as I can tell.
I got some credit cards and used them, and they come out fine. I also got the flat sides of plastic containers and tried them out and they cut out fine as well, and unlike the sheets of plastic that you get with the Guitar Plectrum Punch if you find your own plastic you can get different widths to your picks and even different shapes.
In one case I used a slightly curved side of a dishwashing bottle and the picks came out with the curve on them, they sound great and are thicker. I don’t really like thin guitar picks.