Commingling only happens at the frontend. For each item the vendor is tracked. Physically they are in different bins and more likely in different warehouses. So

Commingling only happens at the frontend. For each item the vendor is tracked. Physically they are in different bins and more likely in different warehouses. So if you buy a product and it turns out fake, Amazon can trace back the vendor for the particular item you received. It helps with inventory availability with the cost of risk of bad customer experience in case of fraud.