I once had a graded card shipped to me in a cut down priority envelope that was nothing more than the front and back of the envelope loosely taped together. I could see the slab through the sides of the "envelope" and it still arrived on time and undamaged. I don't know how the seller was able to ship it with tracking and not be told to put it in better packaging.
I have also had several cards shipped in nothing but a plain white envelope and only a penny sleeve and a sheet of paper for protection. They all survived the postal system and arrived faster than most if not all packages with tracking.
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