By using Padded Envelope is the very cost effective way to pack your Device.
However, Bubble wrap and Box{even better with the original device box}👍 are the extra secure package for mail-in repair.
*Tip: Try to shake the Envelope or Box after packing, if you hear some sound🚫 or something is moving around🚫 internally, Please pack them up again.
*. You can have some idea from What Mail-in Repair Centre Team Does in terms of the secure package. Please check it out below.