When you sell products through Amazon FBA (ship to Amazon, and they sell and ship it with the Prime logo), they give you the option to commingle inventory for most products if they have a UPC on them. They scan the UPC and then credit your account with 1 of the product. Your product is then mixed in with the rest [0].

[0] https://www.amazon.com/gp/help/customer/display.html/?nodeId...