[fujiters]

They could require the vendors attach the stickers to uniquely determine the shipment. Upon receipt, just scan to ensure the barcodes match/aren't duplicates. They already require vendors using Fullfillment by Amazon to have their shipment contain a shipping summary. Adding per item barcodes wouldn't be a huge change (and Amazon itself wouldn't bear the cost).

[reaperducer]

You're right. The items have to be scanned for inventory management anyway when they arrive and are put in the bins at Amazon. Might as well just scan a second bar code to note where it came from. Or do it as a combined QR code. Lots of room for extra data in there.