It's a good way, as long as the person you're dealing with is on the up-n-up.
Personally speaking from anecdotes, the U.S mail system is fine depending on your exact location- in more questionable places you can try a P.O box or the new 'usps lockers' that are in big cities.
And make sure that nobody can tell there's money in there from the outside with a naked eye. Mail is scanned sure, but just put some extra pieces of paper on the outside...
When you remember the fact that some stuff requires sending your documents through the mail (passport stuff if you don't wanna go in person, filing taxes by mail... ) you either gotta trust the mail system or bake in the chance for shit to go wrong.
My personal experience doing cbm trades has been positive, however finding someone to do regular trades with has been difficult. Of course the price of shipping is annoying too, but such is life.
As someone who doesn't wanna deal with getting online bank accounts shut down, cbm is p much my only option ;w;;