I once sent a beer coaster without envelope and just with an address scribbled on and a stamp to a beer loving friend from a holiday. We both were surprised it worked.
Also in the late 90s I remember my favourite computer mag having a picture of a 5 1/4 inch floppy sent to them. Complete with postmarked stamp. Allegedly it survived the procedure.
Ha! I did that a few times with 3½" disks - address and stamp on the label and slap a bit of tape over the shutter to prevent dust ingress. No issues.
I don't think I'd have risked it with 5¼" floppies though, they were a lot less robust and I can't imagine the franking machines would have been good for them.
Also in the late 90s I remember my favourite computer mag having a picture of a 5 1/4 inch floppy sent to them. Complete with postmarked stamp. Allegedly it survived the procedure.