[Ole]

... When sending a print it might be better to send unmounted. 2 out 4 prints, including the print exchange, have come on mat board that curled into a S-curve that was not retrievable. ...

I agree. The stiff board easily gets crumpled, an unmounted print can survive incredible abuse even if the packaging is totally trashed.

Experiment: Take two pieces of cardboard. Put an unmounted print between. Then bend the "package" enough to crack the cardboard. Unpack; examine print.
Then try the same with a mounted print.

Which would you rather get in your mailbox?