Probably I'm doing the wrong thing, but so far all my paintings have arrived at their destinations intact.
I usually send them without a mount and I put the painting into one of the plastic envelopes you use for filing records etc then I put this into a hard backed envelope that is used to send photographs. Then print in BOLD letters at the top of the envelope PLEASE DO NOT BEND then next to that I always add in inverted commas 'Thank you!' (lol)
This may not be the best way to send your paintings so please take further advice from others before trying the above.