Winner: sony1492 guessed it, weirdly packaged tires. They are not fragile and I’ve never received tires in boxes before, but there’s a first for everything.
These are the suddenly-very-hard-to-find Federal SS595s in 205/60R13 for the RX7. I had to email Federal Tyre about them - they gave me contact info for the sole place they supply them to, where I ordered them over the phone two days ago. Ground shipping from NC to OH is apparently pretty quick.