Just in 2024 I have mailed over a hundred insulators that I have sold. I have only had one broken insulator in my 20 year history of shipping insulators through USPS. I was encouraged to continue this method when early on a collector told me the box was in tatters, but no broken insulators. The method is simple, but time intensive. I build a sleeve within a USPS box, and then wrap each insulator with cardboard. I wrap with the corrugations perpendicular to the round of the insulator. Each corrugation will bend and wrap tight to the insulator. This phot show some high density foam, which is harder to come by and now expensive.