I've personally had about 97% of my books shipped in envelopes (bubble, manila, priority) and they've all arrived to me in tip-top shape.
I hear that anecdotal evidence all the time - it is not my experience with HG books, and I get several shipments in a week. Maybe 80% of the books come through unscathed that way, but no where close to all - that is not an acceptable rate. Envelopes might work great for low grade junk, but not for expensive, HG books. I don't see those u-line boxes working well either, as there is not adequate cushioning around the edges.
I had a $75 book folded in half not three weeks ago, due to being sent in a priority envelope, and it had beefy cardboard.
Again, it rarely costs more to use a box, so why not do it? I'm rapidly writing a bunch of you off here.
+1
had a $400 book placed between thick cardboard get folded in half because the sender used an envelope.
It is good to know who uses envelopes so I can buy elsewhere.
I had a $450 book come in off eBay last month with, I shiiite you not, one piece of cardboard in a manila envelope. I lucked out that time though, as no damage. It was a rare VF GA book too - pretty damm careless. I had a 9.6 copy of X-Men 142 come shattered in half because it was shipped in a padded envelope with cardboard. Only a $100 book but still freaked me out at the time. Wasted another four months finding a like graded copy for the same price.