In my experience: as the box size gets bigger, the treatment gets rougher. Single CDs in bubble wrap envelopes come through fine, as do small hardwall boxes. CDs in medium boxes (10-20 CDs) often arrive with box damage and maybe some torn internal packaging. I once used a shipping aggregator that put 50 CDs in one box, they were so banged up that I had to buy replacement jewel cases.
If you ship CDs, I have only one request: please please do _not_ put bubble wrap _inside_ the jewel case! Some sellers do this and it just destroys those little plastic retainer teeth.
Thanks for the feedback. Bubble envelopes don't really protect CD jewel cases very well, which is why I create my own packaging by wrapping the cases individually in recycled cardboard. Simple and cheap. CDs are amazingly resilient and with a proper polishing machine scratches can be completely removed without changing the data on the disc. It is damage on the non-playing side of CDs which is impossible to recover from.
As a used CD seller who often ships internationally with no complaints, I am interested in learning more about what is behind this statement.