I think some couriers also distinguish between..
brand new part in original (and i guess in theory ‘safe’)unopened packaging
and
second hand part wrapped in bubble wrap (it doesnt matter to them safe this is made)
(royal mail certainly did/do this with batteries..)
which would help explain how manufacturers move their stock around…
large companies would also be able to negotiate beforehand with couriers etc when moving their stock around and put specific insurance in place if needed i guess..?
random bloke turns up at the post office with one parcel and its just a safer bet for royal mail to refuse it or ‘destroy it’ as they will have no idea / guarantee of the packaging safety..
and whilst most people will package stuff well.. ive had some ‘interesting’ packaging in the past from private sellers..!