Some odd-shaped (square) greeting card manufacturers have been putting a butterfly symbol on the envelope on the spot where the stamp goes. Although you can use any stamp combo that adds up to the non-machineable rate, the post office actually has stamps specifically for this rate and wouldn’t you know it, they have a butterfly on them. With the rates changing in January, a new one is going to be issued showing a California Dogface butterfly.
UPDATE: This is tangentially related to the thread about paper sizes which I also just updated. The above link to the USPS mailing template is extinct but here’s an updated one:
in response to my original question, what I think happened was that I mailed something printed on cardstock and it was just a little too stiff to meet flexibility guidelines, thereby defaulting to the postcard limit rather than the envelope limit. Lesson learned, use thinner cardstock.