It must be non-deterministic and "risk based," like Google's authentication.

In my most recent return, it was a cheap item, but the incorrect size, and they told me to keep, discard, or donate it and refunded my money.

IMO Amazon returns are easier than ever. You don't even need to box it up, just drop the item off at Whole Foods. I feel like my local Whole Foods is at least 20% an Amazon returns depot these days.