I’ve initially thought about a standard hole puncher, but it only makes 1 hole at a time (there are 80 holes in the postcard above), and thus requires a lot of time overall because correct positioning relative to the other holes needs to be achieved somehow.
I like @LC-Canada’s pinking shear suggestion! And I love your idea for a do-it-yourself postage stamp-shaped postcard. I’m sure some of the “teeth” on the perforated edges would be bent or torn along their postal journey…but that’s just part of the process, right? Please share your results!
Normally you’d use them with a special roll through tool, but you can put it also upside down and press hard with a knife or a roller etc. These are made in many sizes.
I would strongly recommend shaped scissors over a cutting die if the only goal is to cut the edges of a card into a postage stamp or other edged shape. Although @xxxyyy is I think right that in principle you could use a cutting die without an embossing machine (used with cutting dies), in my experience the pressure an embossing machine exerts onto the paper/die is far more than the average person can accomplish, and I think scissors would give a cleaner and more reliable result.
(full disclosure: I did try using cutting dies someone gave me this way (knife, roller, really heavy slab of marble gone over with the roller…) before I got my embossing machine, and the results were…non-optimal. But maybe others have had better results than I did!)
My mom had pinking shears she used for sewing projects sometimes. They come in various designs and would give you this effect just by trimming the sides of the card.
Now I had to try how hard I press and, not very hard, and actually not press, but how I’d peel a carrot, dragging.
But, scissors might be easier to get. My problem with scissors is, if I try to fix the edges, it doesn’t always cut perfect, or straight (not the fault of the scissors).
Here’s my little bird from a die, made using a knife. The ones with only lines (no holes like in this) are much easier but this isn’t hard either.