calumet carton ( a site sponsor ) sells stay flats
i have often send prints to clients in those.
i don't put the "tabs" in the slots though,
i use packing tape and tape the end shut...
never had a complaint ...
you could also put your matted + baggied print in a large box
filled with packing peanuts ... costs quite a sum of $$
( i have send sheets of foam core / box camera kits this way, and the shipping
is NOT cheap. )