COGS are the costs of goods shipped. These are the costs that Amazon incurred to purchase the items from the vendor. Shipped units means units shipped from Amazon to the customer. This is something in between a sell-in and sell-out metric as the shipped units are the units shipped from Amazon to the customer but the COGS are the costs that Amazon had in buying these products from the vendor. Net received is the net amount received by Amazon. That is, what the vendor sent to Amazon. This is typically called sell-in which means how much Amazon bought from the vendor.