I think you’re right that is the core of the model. Good customers always get treated better than Joe Rando.
But Amazon has more information and uses that too. They know loss rates by zip code or by block by carrier.
Usually items I order for them that are higher shrink risk don’t go through their Amazon distribution network and come UPS or USPS. Anecdotally, I’ve noticed those returns are easier — they probably dump cost on the 3rd party shipper.