My recollection (admittedly worked for Amazon >19 years ago) is that there was never any computational overhead to commingling. In fact, the opposite was true: there was a computational overhead to tracking which vendor a specific piece of inventory of a given product came from instead of assuming that all inventory of that product was fungible.
This affected returns as well. For multi-sourced products, we could never guarantee that overstock or damaged items were returned to the original supplier—only that the product matched. Suppliers complained about this a lot.
This affected returns as well. For multi-sourced products, we could never guarantee that overstock or damaged items were returned to the original supplier—only that the product matched. Suppliers complained about this a lot.