I use a 18x12x6 250 lb plain cardboard box, no markings, no anything. On 1-2 books, I put them back to back and then I use 5 48"x12" strips of 3/8" bubble. I wrap two strips in both directions and then cut the last piece in two and put it at the ends. It fills the box and then two strips of clear packing tape to close it off. Since I tape nothing in the inside, it makes it easy to unpack. As for shipping I use UPS and I have never had any thing damaged. I have shipping over 600 books this way.
The closest call I had was with a 10K book. The box was mangled almost beyond recognition but the book was safe and sound inside its bubble wrap.
I definitely do not write fragile, handle with care, or valuable on any box. In any case, human hands usually don't damage boxes with UPS. Most damage occurs on belts when heavy stuff hits think light stuff.