Bulk discounted presorted shipping is a thing for everyone technically - junkmailers have been doing it before before Bezos was born. It is a matter of baked in fixed transaction costs. In addition to sparing the back end sorting costs they would rather get 1 order of 1000 to process than 1000 orders of 1. Most would rather not bother with the overhead of say a 17 cent checkout fee to pay for groceries and just have it baked in the prices.