I'll chime in here. I'm new but I've been spending $ like a drunken sailor on raw comics and have seen lots of different shipping methods, both on books bought here and on EBay. The only comics that have arrived damaged were mailed using envelopes and cardboard pieces. The "cardboard sandwich in an envelope" method works fine AS LONG AS THE COMICS ARE IMMOBILIZED within the cardboard. If the comics can slide around loosely between the cardboard, all it takes is one good whack somewhere in transit and a corner is now poking out, waiting for the next whack to dent the corner. Most sellers who ship with this method have used blue painter's tape or something similar to immobilize the books safely within the confines of the cardboard. I can't understand why some people who regularly sell comics don't get this.
But then again, I've heard the phrase "I've been sending comics this way for a long time and never had a problem" three different times now, 2 from EBay sellers and one here, so I must be an unlucky guy with an extra clumsy mailman.