Not that anyone actually mails out the limited postcards, but in order to get the special "postcard rate" or mail as a "Letter-rate" from the USPS it needs to be rectangle. Actually now that I am looking into it it couldn't mail as a "Flat" either because it has a minimum 6.125" x 11.5" size, so I guess technically it could not be mailed out in a die-cut shape.
That was what I was thinking, that postcard rate is for a rectangle. I agree, no one mails the limited postcards, so this is a non-issue. For some reason these kinds of questions always seem to occur to me..........
Here's what Wacky collectors look like this morning